Well is that 50% at bronze with less then 20 games played or is that 50% at diamond with 200+ games played as that champion.
Win rate without additional information can be very misleading. As high skill cap champions can be very OP with a lower win rate over all. However when you play 100+ games as that champion you might end with a 65% win rate. But at only 20 games the same champion might have a sub 45% win rate.
A lot more information is needed to determine actual Power level of a champion then just win rate.
Lets look at vayne since you brought her up.
Vayne has a overall win rate of 52%. However when you start looking at number of games and her win rate you get a better idea of her overall strength.
Anyone who has 100+ games with her has a win rate of over 53% which is still a balanced win rate. Vayne has a low skill cap and it shows as players with only 5+ games still have a above 50% winrate.
What does that mean for vayne? It means that she is easy to play and yet still a very strong champion. This doesn't mean she is OP or that she needs nerfs. This will very based on meta.
Vayne has become stronger over the last few patches with nerfs to armor and the new juggernaut items benefiting more from health stacking. These were indirect buffs for vayne.
There are a lot of things that have to be looked at to determine balance and win rate is a important but not All Encompassing variable