Vayne's buff was ridiculously unneccessary.
Patch 7.15, Vayne was sitting at a neat 51.09%, which was the 6th highest ADC winrate at the time. She has a Play % of 9.71, and is within the top 5 most banned AD Carries.
It's just outright depressing whenever riot decide to buff a champion because of it's player base. I mean, here we had diana, who was at a 46% winrate for 5 god damn years (or the patch after her release) before she got some real buffs, and yet red lights go off at riot when vayne hits 51%, because apparently thats underpowered, and needs help.