Ill jump in for him. You need to understand that this question has been answered a million and a half times.
First of all, silver bolts can only target one person at a time, so if there are multiple people diving her (which should always be the case) she will have a very hard time procing it successfully.
Continuing from that first point, she has incredibly low range. And before you start on the "oh shes got 550 range, the average range blah blah blah" you need to recognize that unlike just about every other adc out there, she has NO influence outside of that range. This makes her near useless in siege situations (either defending or attacking) and also makes her an incredibly risky choice in general due to the variety of abilities that can hit her at that range. This is a MASSIVE disadvantage this is mitigated somewhat by shiv/cannon but is a far cry from removing these weaknesses.
She also has absolutely terrible ad, it was nerfed even harder than the other adcs. This means that early to mid game Vayne is at a serious disadvantage in lane; if your team isn't taking advantage of this....well........
About the only point where I would say she is too strong really has very little to do with her; in the late game, because of the lack of true sight, she becomes much harder to deal with than she used to be.