What is "the obvious"? How to find a match?

That the general nicknamed Darth Vader would also have been a disabled male with prosthetic limbs, a lovely voice & consistent clothing style? Prime fencer & famous pilot, yet secretive & shy in public? Absent father of twins? Apt in magic, quick to torture, mass killer, even planning to kill his master? An American role-model for servants of an evil empire, for second chances, finding faith, even multiple religious conversions? Or perhaps just father to boy named Luke?

The information on this is just too sparse to pinpoint only one candidate, or even just a few. Too many possibilities. Too popular as a nickname. For a lot of references. Even when 'a general at the Pentagon at the time with that nickname' is identified, we cannot be sure that it was this person Clarke really meant, as surely that nickname was used a few times more. It has to be considered that this may have been not a real nickname, supposedly giving much insight into the 'general' character of the person he met, but simply Clarke's description and invention, or his method to conceal the identity. In this case perhaps not meaning much more than just 'meeting two persons high up in the list of responsible for this people.'

While maybe two generals, perhaps a certain Abrahamson and one Graham, might now seem the likeliest or even somewhat 'logical' candidates, based on the first being at the head of the SDIO at the time, and the latter writing up the main proposal ideas and chairman of the Coalition for the Strategic Defense Initiative and director of High Frontier, firmly confirming this might not reveal much either, and here is why:

In all seriousness, the amount of attributes that this pop culture icon evokes instantly — even in those who never saw the merchandise or the accompanying movies and other media — is just a bit much to match on any Pentagon general to be a perfect fit.

President Reagan delivered his 'Star Wars Speech' on March 23, 1983, without naming either the announced program as Strategic Defense Initiative (SDI), nor did he call it "Star Wars", nor did he name any general as being in charge of that programme.

The timeframe to consider is Star Wars came out in 1977, SDI was announced in 1983, Clarke wrote his piece in 1986.

In fact, when Reagan was given that idea for SDI in February 1983 by the Joint Chiefs of Staff, Admiral James David Watkins led that push. Watkins is remembered by Wikipedia in the relevant time frame for banning beards in the Navy and his work on President’s Commission on the HIV Epidemic. At the same time another military man with high rank and status related to formulating this programme in a leading role was John Marlan Poindexter, again an admiral, but also nuclear physicist, who Wikipedia does not even connect to SDI but just to "multiple felonies", mainly the Iran-Contra-Affair in the timeframe we want to look at.

Despite that the author Clarke directly mentions not the National Security Council members at the time, or any high ranking military, but explicitly a "Pentagon General", we still should not dismiss Clarke using these terms in a figure of speech manner for the just mentioned possibilities. If however the hints given were meant as or simply were accurate, then one general with the Department of Defense moves into focus:

1. candidate: Abrahamson

James Alan Abrahamson (born May 19, 1933) is a retired U.S. Air Force general who served as a designated astronaut, associate director of NASA and former director of President Ronald Reagan's Strategic Defense Initiative from 1984 until 1989.

The problem here is in the images:

Is that a recognisable villain?
Anti-militarists will probably say yes, quickly. But movie fans?

Or is this a Sith lord with his emperor/master?

When in fact Reagan himself was also seen/shown as Vader:

The problem is that not much seems to connect Abrahamson to Darth Vader as known from the silver screen. One point in favour for this candidate is of course that he seems a fit mostly for the Clarke words: "cheerfully"?

Like it was commented on in the press:

When Reagan announced his initiative in March 1983, the project was immediately dubbed “Star Wars” in the press. The title was a reflection not merely on the improbability of making nuclear missiles “impotent and obsolete,” but on the fact that Reagan in a speech just two weeks before had spoken of the Soviet Union as “the evil empire,” and commentators were still joking about “the Darth Vader speech.” Yet those who later maintained that Reagan took his inspiration for SDI from the movies were not joking at all.

— Frances FitzGerald: "Way out There in the Blue: Reagan and Star Wars and the End of the Cold War", Simon & Schuster: New York, 2000, p 22.

The same book goes on to describe Gorbachev's face being made onto a Darth Vader doll and given to a delighted Reagan by a journalist, as Reagan is said to have responded to that gimmick with "they really are an evil empire…"

In addition, a collection of Star Wars advocates including Edward Teller, Daniel O. Graham, Lewis Lehrman and Gregory Fossedal visited the President ten days before he went to Geneva. An editorialist for The Wall Street Journal, Fossedal, twenty-seven years old, was one of the leading publicists for Graham’s High Frontier concept. Fossedal, by his account, gave Reagan a Darth Vader doll that he said was a likeness of Gorbachev; Reagan thanked him and said, “You know, they really are an evil empire.” (Sidney Blumenthal in WP, 11/22/85.)

— FitzGerald, p619

So, was it Reagan, Gromyko, Andropov, or Gorbachev to whom that nickname was applied, and might be applied best? And what do we gain to declare one of either of options as the correct one? (Cf — Edward Tabor Linenthal: "Symbolic Defense: The Cultural Significance of the Strategic Defense Initiative", University of Illinois Press, 1989, pp 16 & 52. By)

This would not be the first time that Darth Vader references are so ubiquitous that a prompt identification under limited information is very difficult:

It is unclear to me who the gentleman in the Darth Vader costume is supposed to be. It is not one of the usual suspects, George Schultz, Edward Teller, Daniel O. Graham, Caspar Weinberger, William Buckley. Who could it be?

More recently, this universal nickname was used to 'humanise' even Dick Cheney, even in the face of trouts and conservation circles not approving. Mitch McConnell 'embraced it' as well. Russian business man Igor Sechin has it attached, as well as American oligarchs John Malone and Elon Musk. NBA persons Bill Laimbeer and John Thomson, NFL person Jim McMahon had it. People like Michael Crichton were seen morphing into the Darth Vader of climate crisis. A moniker all but ensured to be given away multiple times as well.

Michel Rolland is the Darth Vader of wine, while [Jim Chanos is the Dart Vader of Wall Street] — sharing this title apparently with Goldman-Sachs, or Elizabeth Warren?, or Joseph M Gregory of Lehmann Brothers — and Luc Besson the Darth Vader of French cinema. Surprisingly, some describe Donald Trump as "the Darth Vader of trade", and yet some even claim that "Kissinger was not the Darth Vader of the 20th century"? 45 British officer Mike Jackson, while some say the Darth Vader of the evil British Empire also cannot be determined easily.

Back at the Pentagon, another allegedly nicknamed Darth Vader candidate was Jamie Michael Morin, albeit as late as 2015, he at least fits the bill for his interest in building an actual Death Star.

2. candidate Graham

In connection to SDI/Star Wars only one general is named in Arthur C Clarke's biography:

Clarke went on to describe an early “Star Wars” vision, put forth by General D. O. Graham in his “High Frontier” study, where dozens of orbital fortresses are put into space to intercept enemy ICBMs—at a cost of hundreds of billions of dollars. This was more than a half year before President Reagan gave his pro-“Star Wars” speech in the spring of 1983.

“The two superpowers,” said Clarke, “are both led by intelligent and responsible men, yet they sometimes appear like small boys standing in a pool of gasoline—each trying to acquire more matches than the other, when a single one is more than sufficient.”

His was an eloquent plea for reason, intelligence, and well-weighed decisions. Clarke was against “Star Wars” technology and deployment even before the press had borrowed the Lucas film title or heard of its euphemistic acronym, SDI (strategic defense initiative]. Space was international. It should be a place for cooperation, not conflict, for the challenges of exploration, not war.

and for more needed context

Clarke named his presentation “A Martian Odyssey,’ a title he admittedly borrowed from the American science fiction writer Stanley Weinbaum, whose 1934 story by that name became his best remembered.

Before addressing the subject of cooperative space missions (what he called “technological decency”), Clarke opened his fifteen-minute video with a discussion of his views on Strategic Defense Initiative (SDI; “Star Wars”) weaponry (in one context referred to as “technological obscenities’).

“I have also talked with many of the experts involved — hawks, doves, and those who, like myself, might be classified as anxious falcons,” he said, before telling the senators that, yes, he believed that ICBMs could be intercepted and destroyed by projectile or beam weapons but that a 90 percent success rate would be astonishing: “I doubt if any informed person really believes that such a figure is possible.” But if it were, he went on—if nine out of ten missiles were effectively stopped—the remaining ones that got through would unleash a destructive power equivalent to a World War II every ten seconds: “The result would make ‘The Day After’ look like an optimistic exercise in wishful thinking.”

Four months later, following the well-received premiere of 2010, Clarke found himself in a more conservative setting. A group of approximately forty people met at the home of writer Larry Niven in Tarzana, California. It was Saturday, December 8, and they were all members of the Citizens Advisory Council on National Space Policy, a group organized in 1980 by Jerry Pournelle. The membership consisted of scientists, professors, and aerospace and military men who advocated and promoted a space defense posture for the United States. Most of them were politically conservative and were strong proponents of the military concepts that would become popularly known as “Star Wars” and formally designated as the Strategic Defense Initiative (SDI). Portions of President Reagan’s Star Wars speech of March 1983, in fact, originated from a similar meeting of the council in 1982.

“About fifty of the top experts in the country were in the room,” says Jerry Pournelle. “They were not just space enthusiasts, but people like Max Hunter, General Daniel O. Graham [author of High Frontier], Lowell Wood, Edward Teller’s chief deputy, and so on.

— Neil McAleer : "Arthur C. Clarke: the authorized biography", Contemporary Books: Chicago, 1992. (archive.org, p 301, 325–327)

Somewhat less "cheerful" than Abrahamson, but visually at least a slightly better match?

This prominent airforce general pushing the SDI/Star Wars idea was Daniel Orrin Graham known for — George Lardner: "Gen. Graham's Star Wars", The Washington Post, November 17, 1985.

Given the direct mention of Graham in the biography and the conspicuous absence of any mention of Abrahamson, Graham gets a slight bonus for this. But a confirmation for the Vader nickname was not found.

Further reading on the variable and changing attachments of various movie and character references to Star Wars in:

— Peter Krämer: "Fighting the Evil Empire. Star Wars, the Strategic Defense Initiative, and the Politics of Science Fiction", in: Douglas Brode & Leah Deyneka (eds): "Sex, Politics, and Religion in Star Wars. An Anthology", Scarecrow Press: Lanham, Toronto, 2012, pp63–76.

— Anette Stimmer: "Star Wars or Strategic Defense Initiative: What’s in a name?", Journal of Global Security Studies 95, 3 (2019): pp515-533. doi

— Donald R Baucom: "The Origins of SDI. 1944–1983", Modern War Studies, University Press of Kansas: Lawrence, 1992. (archive.org)

3. On the historical literature, original provenance of the introductory blurb

In the Collected Stories version (archive.org) the quote goes on to reveal:

[…] under the nickname ‘Darth Vader’. I am happy to say that I am on good terms with all of them, even if we don’t agree on what could, and should, be done in this controversial area.

This introduction did not appear in the original publications, like Omni Magazine May 1987 (PDF). Also, this has to be read with the actual content of the story, that features a fictional Pentagon general, which by some chance gives a few clues to narrow this down a bit more via character traits of the military man depicted. The relevant parts:

One person who was not embarrassed by this criticism — or any other — was the able and ubiquitous director of the BDIO. General Isaacson had made his formidable and well-deserved reputation as a Pentagon trouble-shooter: perhaps his most celebrated achievement was the breaking up of the sinister, Mafia-controlled ring that had attempted to corner one of the ‘most lucrative advertising outlets in the United States — the countless billions of sheets of armed-services toilet tissue.

It was the general who harangued the media and arranged demonstrations of the still-emerging BDI technology. His presentation of gold — well, gold-plated — tie clips to visiting journalists and TV reporters was a widely acclaimed stroke of genius. Not until after they had published their fulsome reports did the media representatives belatedly realise that the crafty general had never said in as many words that the gold had actually come from the sea. By then, of course, it was too late to issue any qualifications.

At the present moment - four years after the President’s speech and only a year into her second term - it is still impossible to predict the BDI’s future. General Isaacson has set to sea on a vast floating platform looking, as Newsweek magazine put it, as if an aircraft carrier had tried to make love to an oil refinery.

This brings Graham again closer to be the 'chosen one' for this introduction paragraph, as Abrahamson was said to be by then more intent on selling SDI to the public as 'things that work', while Graham's ambitions and tactics remained much more stubborn and clinging to the 'original vision' of his High Frontier, although he was by then retired from the Pentagon. (Cf — William M. Knoblauch: "Selling “Star Wars” in American Mass Media", in: Henrik G. Bastiansen, Martin Klimke, and Rolf Werenskjold (eds): "Media and the Cold War in the 1980s. Between Star Wars and Glasnost", Palgrave Studies in the History of the Media, Palgrave Macmillan: Cham, 2019, pp19–42. doi)

4. Worden — remote possibility with the actual nickname documented

Given that the hint from the introduction of Clarke's short piece was written after 1987 and closer to 2000, another man might fit the bill, despite him being only Major in the early 1980s, making General only in 2000:

Pete Worden is documented as :

In the 1980s, he was an early advocate of President Ronald Reagan’s Strategic Defense Initiative, an unpopular stance that earned him the sobriquet of Darth Vader in space circles.

— Andrew Lawler: "Pete Worden 'Ames' for the Moon and Beyond", Science, Vol 315, 19 January 2007. doi

While his life dates seem to make him 'too young' for a 'perfect' match, he did have that name attached, and more over the snippet above clearly showcases how almost arbitrary and widespread this nickname was and is handed out. These conditions were just too easily met in real life to make this easily answerable.

5. Wood, mind you, non-General, meeting Clarke and described as Vader:

Lowell Wood was also mentioned in Clarke's biography by name and acquired a reputation, while being apparently no real military general, but certainly yet also another bearer of the nick:

How could it be "only physics," when there was such black humour about death and destruction, Russian spies; and when their boss was compared to the evil character Darth Vader from the film "Star Wars?" "Darth Vader" was Lowell Wood: one of the most abusive personalities I have ever encountered. He would set out major scientific challenges, described as "trivial," and press the young scientists to deliver on unrealistically short timetables. Failure to produce would place the scientist at the receiving end of brutal, sarcastic insults and renewed pressure. There is no doubt that Wood was able to create a powerful esprit de corps among the 0-Groupers, but with a cruel edge to it.

— Josephine Anne Stein: "Scientists, Engineers and the Anns Race", in: John Hassard, Tom Kibble & Patricia Lewis (eds): "Ways Out of the Arms Race. From the Nuctear Threat to Mutual Security", World Scientific: Singapore, New Jersey, 1989, p24. doi