Is there any first-hand evidence of intentional spreading of disease among natives in the Pacific Northwest?

No. There is no evidence of the intentional spread of smallpox into the pacific northwest.

That would require previous contact with the intent to decimate this population. There is no logical reason for this to happen. The most common interaction with the native populations on the western coast at this time were for trade purposes (see all the questions concerning the Spanish missions of Alta California and Russian American companies fur trading). You would not want to kill off the labor force you are using to collect these furs, as mentioned in comments.

Concerning your theory that the disease would have had to have been brought 'around South America' we can show historically that this would not have been necessary. Smallpox outbreaks were raging in the colonies throughout the time of the US Revolutionary War,as well as in Mexico which suffered a major devastating epidemic during this same time frame. The Wikipedia page on the 1775–1782 North American smallpox epidemic doesn't put this together well for the purposes of this question, but we can look at a couple of excerpts:

By 1779 the disease had spread to Mexico and would cause the deaths of tens of thousands. At its end the epidemic had crossed the Great Plains, reaching as far west as the pacific coast, as far north as Alaska and as far south as Mexico, infecting virtually every part of the continent.

One of the worst tragedies of the pandemic was the massive toll it took on the indigenous population of the Americas. The disease was likely spread via the travels of the Shoshone Indian tribes. Beginning in 1780 it reached the Pueblos of the territory comprising present day New Mexico. It also showed up in the interior trading posts of the Hudson's Bay Company in 1782.2 It affected nearly every tribe on the continent, including the northwestern coast. It is estimated to have killed nearly 11,000 Native Americans in the Western area of present-day Washington, reducing the population from 37,000 to 26,000 in just seven years.

Bits of this entry hint at the spread from the regions of Texas and Mexico, into the central plains, and finally into the Pacific Northwest, via contact from one culture to the next. No contact with colonial settlers or traders was necessary once it began.

Though it is not clearly spelled out in the wiki entry, the details of this transmission have been researched by one of the sources listed for the entry, by Dr. Elizabeth Fenn, a historian who did her thesis on this epidemic and published the book Pox Americana: The Great Smallpox Epidemic of 1775-82 She details the spread of this disease northwards, through trading or other inter-tribal contacts. You can watch a lecture presentation on YouTube where she discusses these events (she starts with Vancouver's discovery), and though she has no primary sources for the final spread into the Northwest, she theorizes a final connection from the central western states into the pacific northwest via the Columbia river corridor.

We can look at one more source concerning what Vancouver found, Vancouver himself. From his A voyage of discovery to the North Pacific ocean, and round the world;, pgs 229-230:

We landed not far from the largest rivulet, where we found a deserted village capable of containing a hundred inhabitants. The houses were built after the Nootka fashion, but did not seem to have been lately the residence of the Indians. The habitations had now fallen into decay; their inside, as well as a small surrounding space that appeared to have been formerly occupied, were over-run with weeds; amongst which were found several human skulls, and other bones, promiscuously scattered about.

This description indicates a village which had not been in use for some time. The tragedy which had struck here was not new in May of 1792, but must have happened quite some time earlier to find remains in the state of skulls and bones scattered about.

In conclusion, considering the steady expanse of this infection through the native american populations, and the timeline involved with an ongoing epidemic in the 1780s, It is quite probable that the depopulated villages found by Vancouver in 1792 were victims of the natural spread of this epidemic. No blankets involved. No trips around South America necessary.

(Note that Fenn does discuss the Fort Pitt event concerning the blankets in questions near the end of the video. )