Tank meta.
Eh, tank meta's been more hyped up than it's really worth.
I'd go more with several marksman receiving nerfs at the same time Vayne received buffs.
She's currently sitting in the highest AS per level gains (4%) where she used to be average (~3%), and her ultimate grants 30/50/70 AD where last time I played her extensively her max-rank ult only offered 55 and later 50 AD, for most of the time I mained her less than a Janna bubble.
It's also worth pointing out that in itemization when she started - typical marksman build had pretty lacking %AS options to start a game with, and Vayne was largely regulated to running with:
just to survive lanning, while not putting the raw AD to nearly as good of use as most (which went all the way up to 100, and was amazing for the AD burst on champions like

Do a really solid job at dealing with most of Vayne's early-game itemization issues now, along with giving her a decent, continuous spike when several of her rivals are saving for that BF sword. Her early-game weakness is pretty null with how good her early itemization path is.
Also worth noting is a thing that's gives Vayne even more potential through itemization than she previously had.
With all these factors together - it's not much of a surprise Vayne is doing well.
TLDR: Vayne got buffed, her items got buffed, her rivals got nerffed, her rival's itemization got nerffed.