It should be Kog'Maw.
We're talking about a voidling creature that constantly spits out an otherworldly vomit that burns everything. It would make thematic sense that this vile shit should destroy ANYONE who's afflicted by it for too long, since it's an alien weapon that doesn't exist on Runeterra.
And balance-wise, it's more fitting to have such disgusting damage on an immobile champion that can only protect itself with a slow field. Its damage could be altered to do %health true damage on each attack, but less than the current values for Silver Bolts, as they're obviously every three hits.
I'd be fine with Vayne having %health true damage if she wasn't a strong, mobile duelist with better self-peel than Kog, but unfortunately, all of it is true.
Perhaps we can alter her silver bolt proc to ignore a percentage of enemy armor instead? Tanks can still build health, and it's not like Vayne players are going to stop buying Blade of the Ruined King anyway.
Praise this idea, flame it, I don't care. I just want there to be some semblance of balance among ADC's.
EDIT: Changed Silver Bolts idea to ignore a % instead of all enemy armor; that might be just as broken.
EDIT 2: Okay so maybe some of you aren't reading this right.
I'M NOT KEEPING HIS % HP MAGIC DAMAGE IF THIS CHANGE HAPPENS. IT WILL ONLY BE % HEALTH TRUE DAMAGE. NO MORE AP RATIO.
Jesus christ, of course if he has both types of damage like that he'd be broken.