My stats (from the website of the public library where I work) for the entire second quarter show Eight lagging a couple percentage points behind Vista, but since it's increasing, that may no longer be the case at the end of the quarter. (The traffic volume on our site doesn't, in my judgment, really support looking at very much less than a quarter at a time, so I generally don't.)
But yeah, what you said: Vista has been on the decline here since early 2010. It peaked at about 25% usage share (a little more than half of what the then-leading version, XP, had) sometime in the second quarter of that year. 25%, incidentally, is several (perhaps five) times as much share as Eight has now.
It's also worth pointing out that Vista has less than two-thirds the usage share that XP has, and XP has less than one-third the usage share that Seven has. (Again, that's all for the whole second quarter.) Eight has roughly a tenth as much share as Seven.
Also, Eight is in a dead heat with Snow Leopard (Eight comes out slightly ahead of Snow Leopard on my numbers, but statistically, at our level of traffic, it's a wash) and has about twice as much usage share as Jelly Bean.
I suspect Eight adoption will pick up a bit when 8.1 comes out. In a couple more years, it may even manage to attain a higher usage share than XP.
Eight does not, however, appear to be much threat to Seven yet.