She's not OP, or at least in terms of LCS capability. However, I can imagine her being a problem for ladder, specifically low ELO. This applys not just to Vayne, but other champions like Yasuo etc, any of those supposed face-smash-keyboard champions. The truth is, these champions are very strong when they are fed and generally have specific weaknesses to counter act this. A 10-0 Yasuo can beat nearly any other champion when he's 2 items up. He doesn't need to do fancy E dodges, Q pokes and W blocks, even though he can. While he can be played well, it's not necessary to because how well he scales into items. Vayne is the same problem, and even Yi. I actually think Yi is almost as complex as Riven, but like each other, when fed, face-smashing is all you need.
This problem of low-skill, high-efficiency generally isn't a problem. The problem is that teams and low ELO players don't know of, or don't know how to abuse the weaknesses of these champions. So, inevitably, they get farmed, and trade kills all game, you now have a 10-12 vayne against a 9-6 Lucian, and because you've been dominating them in the lane phase, you all the sudden think you can duel her alone.
The key to beating Vayne is to abuse her weak early game, of which almost all other marksmen can beat her pre6 in a duel, and often post6 with a competent support or normal lead in cs as you should have, then you take your lead to dragons, towers, other lanes, or skirmishes.
However, I do agree with your sentiment a little, but it's not Vayne's problem, it's moreso soloq, and low ELO players. This is why you should always play very strong scaling champions the lower ELO you go because nearly no one knows how to win games off early game power spikes and leads.
So to sum it up, yes, there's a slight problem with Vayne, and a few other champions, but not LCS'wise, so play scaling champions more if you find you or your team cannot win off leads.