Why Vayne is (at least to most people) OP
Seemingly, most people have a problem with her true damage.
However, the solution is actually quite simple. And that is buff the other ADCs, make them more relevant.
Ideally, Twitch and Lucian should be very good ADCs. Twitch's stats are low and the range on some of his abilities are lackluster. Lucian has been nerfed too much and can't secure kills as well as Vayne or even Draven.
Make the other ADCs stronger, and Vayne won't seem as bad. In theory, all the ADCs should serve the same purpose of getting kills and being the team's ace in the hole. But, we need to make each of the very strong at doing a certain thing. There is no reason for one champion to completely outmatch another. This game should be based only on skills and tactics. The ADC who has the best tactics and uses his characters abilities to their advantage should come out on top.
Just my two cents.