do you know who really needs HP% true damage?
towers.
they need it not 
towers.
they need it not 
vaynes kit is in a good spot
I don't mind towers dealing true damage I do mind % hp as that removes a reason to be tanky at all. And would mean a adc can tank a tower just as easy as the tank and even easier with a healer as the healer would heal a larger % of their health.
do you know who really needs HP% true damage? towers.
I really don't see why towers should have that. They already have armor pen and ramp up in damage to begin with, and they already punish melee champions much harder than ranged ones during the lanning phase.
I mean - such a change is basically saying:
Screw you guys, you can't kill someone under tower until late game. Or even help your allies do anything involving you and a tower.
Which is really awkward considering
Can all kill you under your tower while taking 0 damage. Or at most 1 tower shot.
Vayne has literally been the same since release, she isnt going to change now that the meta has shifted a bit, she will stay until the meta shifts more and she is no longer circle jerk worthy again.