I don't necessarily think she's too strong early game; I think the problem is that, if you look at the past (or if you were present, I don't personally know) anyone considered a lane bully got nerfed. Graves has been nerfed, Corki has been nerfed, Lucian has been nerfed, many bully supports, etc.. Vayne would be just fine if they let lane bullies actually be lane bullies in the bottom lane, but every time they do people complain on the forums that an ADC, a champion who is naturally going to be strong late game, is able to excel in the early game. This has fostered a situation in which people like Vayne, who would hate to be against an actual lane bully, have a safe haven to reach their late-game mostly unchecked.
I honestly think the problem is less to do with Vayne and more to do with the balance philosophies of the game itself allowing someone like Vayne to have a poor early game be acceptable by way of making everyone who could contest her equally bad because of the scaling nature of the class. This is like if Spectre (DotA2) had her godly late-game and poor early-game combo remain intact, but every lane bully was mostly knocked down to her level because otherwise it's "oppressive."