How does the story of General Siha's Meal for the Buddha at Vaisali vindicate the Buddha?

Just to complement Chris' response, the sutta which i think is the final authority with regard to Buddha's position on meat consumption is Jivaka sutta (MN 55)

I like the next wiki page on buddhas diet (which even mentions siha story discussed here ) which shows that even if the buddha did eat meat (and its not 100% sure) it was very rare :

http://www.dhammawiki.com/index.php?title=Diet_of_Buddha

There has been some controversy about the diet of Buddha, if he ate meat or if he was strictly vegetarian. The first Precept states no killing or causing to kill living beings, but at the same time, monastics are expected to take what is offered them, living by the Threefold rule. Contents

1 The Buddha's final meal
2 Meat eating
    2.1 General Siha
3 Vegetarian foods
4 Vegetarian foods served to former buddhas
5 The Three most important meals
6 Scientific sample
7 See also
8 References

The Buddha's final meal

At his final days in the parinibbana sutta, the food that led to his death was at one time translated as pork. The terms have been translated as “pig’s truffles” which was originally mistranslated as pork. Modern scholars including, Arthur Waley, K. E. Neumann, and Mrs. Rhys Davids have corrected this to “the food of pigs” which are mushrooms. Today, the majority of Buddhist scholars agree that the Buddha ate mushrooms, which may have been poisonous and led to his death at the age of 80. Or it could simply have been the size of the meal that led to his death as there is evidence that the Buddha was already suffering from digestive problems well before eating the final meal (from previous suttas where the Buddha was ill and then recovered). However, the Buddha eats from the dish and requests for the remaining amount to be buried, apparently knowing that the food was in some way tainted and not simply a large meal. This suggests that the food was in some way not fit to eat, such as the wrong type of mushrooms. From the Digha Nikaya, Mahaparanibbana Sutta, no. 16:

19. Thereafter the Blessed One spoke to Cunda, saying: "Whatever, Cunda, is left over of the sukara-maddava, bury that in a pit. For I do not see in all this world, with its gods, Maras, and Brahmas, among the host of ascetics and brahmans, gods and men, anyone who could eat it and entirely digest it except the Tathagata alone."

And Cunda the metalworker answered the Blessed One saying: "So be it, O Lord."And what remained over of the sukara-maddava he buried in a pit.

20. Then he returned to the Blessed One, respectfully greeted him, and sat down at one side. And the Blessed One instructed Cunda the metalworker in the Dhamma, and roused, edified, and gladdened him. After this he rose from his seat and departed.

21. And soon after the Blessed One had eaten the meal provided by Cunda the metalworker, a dire sickness fell upon him, even dysentery, and he suffered sharp and deadly pains. But the Blessed One endured them mindfully, clearly comprehending and unperturbed.

Even today, in modern times, it sometimes happens that people prepare and cook the wrong type of mushrooms and then become ill and pass away. And the illness and passing happen very quickly. The food may have been the wrong type, not from poisoning, but simply by accident and therefore, no intent to harm by the giver of the food, Cunda. The Buddha continued showing his compassion for all by prasing the gift of the food and then later teaching Dhamma to Cunda, others, and even one last ascetic who came from far to see the Buddha to learn the Dhamma.

Further evidence that the Buddha did not eat pork can be seen in the fact that Cunda was a blacksmith, the one who offered the final meal to the Buddha. The three highest castes do not eat pork or other foods from pig meat. As a blacksmith, he was a member of the third caste and therefore, could not have prepared pork.

K.E. Neumann, in the preface to his German translation of the Majjhima Nikaya, quotes from an Indian compendium of medicinal plants, the Rajanigantu, several plants beginning with sukara.

Other vegetarian possibilities mentioned by scholars and early Theravada Commentators include a medicinal plant, a yam, or a tuber. Dhammapala, in his commentary to Udana VIII.5, suggests that it refers to young bamboo shoots trampled by pigs (sukarehi maddita-vamsakaliro). Meat eating

There appears to be one place, and apparently only this one place, where he is described as eating meat. At AN III. 49 it mentions that the Buddha was once served sukaramamsa (Pali) with jujube fruit. The term mamsa = meat or flesh.

The sutta mentions that the Buddha ate "out of pity" apparently suggesting that he wanted to please the layman by accepting his food. General Siha

There might be one other case where the Buddha ate meat. After hearing a Dhamma talk by the Buddha, (the military commander) General Siha addresses one of his staff and tells them to go to the market to purchase some meat for the Sangha. The Niganthas (Jains) complain that the Buddha and his monks will be eating meat from an animal killed for them. But the Buddha explains that no animal was killed specifically for the meat according to the 3 fold rule. The passage does not specifically state whether the Buddha ate the meat or not. Later it is reported that the "pure stainless eye of the Teaching appeared to the general Siha seated there itself; Whatever arisen thing has the nature of ceasing" (Anguttara Nikaya 8.12 / AN IV.185) which implies that he attained stream entry (sotapanna). This suggests that lay people can purchase meat without violating the First Precept since it is an indirect connection and no specific animal ordered to be killed. However, upon closer examination of the Suttas, we find that this passage completely contradicts other teachings of the Buddha against killing, causing to kill, the Buddha's opposition to animal sacrifices, slaughter houses, butchers or any other livelihood in the business of meat (Anguttara Nikaya 5.177). And in another Sutta the Buddha reports that soldiers are not headed to heavenly realms and in fact have rebirth to lower realms (Samyutta Nikaya 42.3). A stream entrant has no more than 7 more rebirths and they are always in higher destinations of human or deva realms. Therefore, General Siha could not have been a stream entrant and / or the passage about him purchasing meat must have been added by later writers of the Tipitaka.

It turns out that the mentioning of General Siha purchasing meat must have been added later to that Sutta. This is very likely when we look further into the Tipitaka and see that General Siha was a Jain up until the time of conversion (AN 8.12 / AN IV.185). The Buddha even advised Siha to continue providing alms to Jain monastics. The Jain religion requires vegetarianism and as such, Siha could not have been interested in meat and there is nothing mentioned at the time of the Buddha's sermon that would all of a sudden want to make Siha a meat purchaser and meat eater. Vegetarian foods

Yet in spite of the one reference to meat, there are numerous other places where it mentions what the Buddha ate and they are vegetarian:

Thick milk-rice porridge and fresh ghee (Udana 4.3)
Wild rice (Majjhima Nikaya I. 78)
Rice with grains (M. 247), (Majjhima Nikaya 36.33)
Barley meal honey balls (Vinaya I. 4)
Choice rice with curry (Majjhima Nikaya II. 7)
Rose apple, mango, myrobalan fruits (Vin. I. 30)
Steamed barley and rice (Vinaya, Suttavibhanga 1)
Lettuce and fruit salad (Vinaya, Suttavibhanga 33.1)
Vegetables and rice (Vinaya, Cullavagga VI 4.1)
Milk (Vinaya, Mahavagga VI 34.20)
Conjey (watery rice porridge) and honey lumps (Vinaya, VI 24.3-7)
Vegetables and bread (Vinaya, VI 36.7)

In the Khuddaka Nikaya, Vimanavatthu, there are stories of meritorious deeds done by lay people who then ascended to a deva (heavenly) realm. Many of the meritorious deeds were giving alms food to the Buddha or one of his monks. Here we find several examples of only vegetarian food either given to the Buddha or one of his chief monk disciples, Moggallana or Sariputta:

Ghee, honey, sugar, rice, milk (Vim. 1.5)
Molasses (Vim. 4.2) and (Vim. 6.4) and (Vim 6.5)
Rice-crust (Vim. 2.10)
Cake (Vim. 3.1)
Sugar-cane (Vim. 3.2) and (Petav. 4.5)
Rice-crust, soups and curries (Vim. 2.3)
Mangoes (Vim. 6.3) and (Vim. 6.5) and (Vim. 4.8)
Rice custard / pudding (Vim. 6.7)
Rice gruel (Vim. 4.4) and (Vim 4.5) and (Petav. 3.5)
Rice, cane-juice, sugar cane (Vim. 5.12)
Rice gruel and mangoes (Petav. 4.12)
Beans, grains (Vim. 7.6)

Vegetarian foods served to former buddhas

The Buddha of our time is the teacher who rediscovered the Dhamma. In previous aeons, there were Buddhas who taught the Dhamma, according to Buddhism.

The bhikkhuni Rohini was an arahant (enlightened) and in a previous life, from a prior aeon, she served "sweet cakes" to a former Buddha (Thi. 67). The Three most important meals

Main article: 3 most important meals to a Buddha

According to Buddhism, the three most important foods served to the Buddha were the final meal (discussed above), most likely mushrooms, the meal just before enlightenment, which was the milk rice served by Sujata, and the meal right after enlightenment, which was barley meal honey balls. All three of these meals were vegetarian. Scientific sample

A scientific sample is one that is done without any bias toward selecting the things to be studied or evaluated. The passages mentioning what the Buddha ate appear to fall into that category as they are spread out through the Pali Canon and refer to other teachings, not about diet and thus, appear to be random mentions of his diet. As such we can use the above as a representative sample. If we count all of the above plus the last meal, the meal Sujata gave to the Buddha and the mention of meat above, we come to: 35 vegetarian meals and 1 meat meal. This results in a diet by the Buddha that is 97% vegetarian. This is the equivalent of eating vegetarian all year except for 10 days per year which is less than one meat meal per month. Such a person even in modern times would most likely be defined as a vegetarian who makes some rare exceptions as may be necessary for social reasons. Of the 35 vegetarian meals 74 percent (26) were vegan (no meat and also no animal products). Even if we include the possibility of there being a second meat meal in the passage about General Siha (see above), then it still calculates out to 95% vegetarian (35 out of 37) diet of the Buddha.

Here is a version of the story as told by Ajahn Brahmavamso ("a British Theravada Buddhist monk"), in an an essay titled, What the Buddha said about eating meat:

Once, a rich and influential general by the name of Siha (meaning 'Lion') went to visit the Buddha. Siha had been a famous lay supporter of the Jain monks but he was so impressed and inspired by the Teachings he heard from the Buddha that he took refuge in the Triple Gem (i.e. he became a Buddhist). General Siha then invited the Buddha, together with the large number of monks accompanying Him, to a meal at his house in the city the following morning. In preparation for the meal, Siha told one of his servants to buy some meat from the market for the feast. When the Jain monks heard of their erstwhile patron's conversion to Buddhism and the meal that he was preparing for the Buddha and the monks, they were somewhat peeved:

"Now at the time many Niganthas (Jain monks), waving their arms, were moaning from carriage road to carriage road, from cross road to cross road in the city: 'Today a fat beast, killed by Siha the general, is made into a meal for the recluse Gotama (the Buddha), the recluse Gotama makes use of this meat knowing that it was killed on purpose for him, that the deed was done for his sake'..." [1].

Siha was making the ethical distinction between buying meat already prepared for sale and ordering a certain animal to be killed, a distinction which is not obvious to many westerners but which recurs throughout the Buddha's own teachings. Then, to clarify the position on meat eating to the monks, the Buddha said:

"Monks, I allow you fish and meat that are quite pure in three respects: if they are not seen, heard or suspected to have been killed on purpose for a monk. But, you should not knowingly make use of meat killed on purpose for you." [2]

There are many places in the Buddhist scriptures which tell of the Buddha and his monks being offered meat and eating it. One of the most interesting of these passages occurs etc.

I think that answers your questions: I think it's basically that meat was bought from the market, instead of animals "killed on purpose for him".

You could probably find more references to the story using this Google search: vinaya siha meat

See also this explanation of meat-eating, in The Bhikkhus' Rules — A Guide for Laypeople.

Another explanation I read (I don't remember where, at the moment) is that the Buddha wanted monks to be easy for the laity to support: i.e. not too demanding/fussy about what kind of food they will accept.