An alternative view on Vayne
http://champion.gg/champion/Vayne/ADC
Vayne has a 52.28% win rate, and a 35.08% play rate. Using these numbers, lots of people have been calling her overpowered, as someone who is played with a 35% play rate should usually not have a win rate over 50%.
However, look at her winrate by games played and her playerbase experience distribution for one moment. According to those, over half of all Vayne players have over 50 games of experience with her. That means that every second Vayne that's in your game is going to be a highly experienced Vayne.
Her win rate at 15-50 games is 51.85%. Riot has stated that they regard 52% at 30 games played to be a balanced state for a champion.
Players who are experienced with a champion SHOULD have a >50% win rate. 53% sounds to be very balanced for someone who mains a champion. The thing with Vayne is just that there are now so many Vayne's that are highly experienced that she seems much stronger than she actually is.
I'd actually say that this issue we have now is a leftover from the tank meta - when the tank meta was at it's height, players would start to pick up Vayne and learn her in order to counter the number of tanks. Why was Graves so common in the tank meta? Because he was a leftover from the previous meta where Graves was very powerful. It takes time to learn and get really good at a champion.
Vayne being popular and strong now is a mix of leftover and the fact that she's not as bad as other ADCs against assassins thanks to her tumble, invis, and condemn, whereas other adcs need to rely on their team to save them. And sure she may be a pain in the ass to play against (R.I.P. my bubbles) and she may be somewhat strong at the moment, but I don't think she's overpowered. Just strong.