Are champions with %hp true damage unhealthy for the game?

No more than champions with targeted crowed control are unhealthy for the game.

vayne in particular, yes.

the others no, because their % true damage has gameplay counters. silver bolts is unconditional. she gets it for auto attacking any target infront of her. there is no decision making involved.

If the numbers are right, no. I think true damage has a place in the game. %hp true damage lets you threaten tanks without delivering too much damage to a non-tank squishy. So it helps keep tank metas in check and people hate tank metas.