I agree that % Health True Damage is mechanic/damage type in the game that is inherently toxic because it lacks no noticeable counterplay. It especially should not be allowed on an ADC like Vayne, who is hyper mobile(2 second tumble at rank 5 and stealth with a knock bock is pretty mobile for an AD when we have ADCs like Kog, Varus, Jhin, and Jinx who have no dashes and are meant to scale harder than her) and has great kiting potential because of it. How you deal with ADCs has always been a matter of CC or stat checking; what are you meant to do when stat checking doesn't work and they have the mobility to dodge your CC?
I've always thought that Vayne and Fiora for that matter, were exceptionally unhealthy and poorly designed champions because their entire kit revolves around something that is unhealthy. Usually the counter play to them would be to shut them down earlier but I have to ask,"What is there to shut down?" Vayne doesn't even have to build damage to destroy tanks; she literally only needs AS in order to come back later in the game. I've destroyed so many Vayne's early but it didn't matter because their true damage meant,"Yeah, I have two less items then you buy I'm still demolishing your tank and then you because I get massive amounts of free AD off my tumble! c:"
Vayne is inherently designed in a way that means she can easily play from behind and that just negates the fact that her early game is terrible. :/ She, as an ADC, can literally build anything as long as it gives her AS and still do fine.
I wish they'd look at % Health True Damage and do something about it.