Yeah...
That's a bad idea.
Normally having such an underwhelming passive would warrant a buff, but its VAYNE we are talking about here. As it is right now, vayne is incredible at punishing overly aggressive tactics and catching out of position opponents. When fighting Vayne, retreat isn't an option as her passive, q, and ult ensure that she will be able to follow.
That said, as it is now vayne has two weakness that prevent her true damage from dominating:
1.) She's a typical hyper carry, meaning she has a really bad early game and trades poorly against anyone that knows what they are doing.
2.) Her only means of disengage also happen to be her engagement tools.
The latter is the more relevant, because this new passive would remove the weakness entirely. The thing is, Vayne can get into fights but as you said, she can't get OUT or kite with this passive. This means that vayne has to rely on her q and e for safety, but if she uses them for that, then she can't stun with her e nor capitalize on the damage buff from her q. This new passive would make vayne much safer and given that she is a hyper carry, she needs risk to justify the massive reward percentage health true damage brings.