I can distinctly remember the precise buff that put Vayne over the edge
BoRK's distance increase.
Before she'd have to be within 300 units - generally a dangerous place to be, and it'd take her longer to catch people or it'd put her in a bad spot.
Now she rolls in, autos, slows you at 550 units, perma-chases you with her passive, and that's it.
If she's winning a fight, you can't escape.
But if she's losing a fight, it's BoRK, one E and a roll away and she's gone.
You basically turned her into the snowballingest champ in the game by requiring people to fight her or die trying to run, yet she's so effective at dueling that turning isn't even really an option so much as a "might as well just because". It's Akali all over again - she gets fed off of just finding people roaming.
Addition: To everybody pitching up the armor nerfs making her more prevalent of an ADC: she deals % health true damage.
Why would armor nerfs increase her viability more than her fellow ADC's, and why would that be her tipping point if she was already absurdly popular before the nerfs ever hit?
This also applies to the tank nerfs, and to the AD item buffs she wasn't prevalent until patches later. Please stop pitching out things that literally just helped the ADC role as a whole and not exclusively Vayne who seemingly went from "late-game hypercarry" to "roaming snowballing lane-bully" within a short span of time with the most logical deduction being that, since BoRK is arguably all she needs, it would be safe to say that an item uncommon on most other ADC's would have been responsible for her suddenly having relevance were it to be buffed.
Thank you for riding the logic express, have a pleasant day.