One thing I like about items: their passives. Am I the tank and enemy Vayne is someone to be scared of? Maybe an attack speed aura debuff (frozen heart, randuin's) will help. If she dedices to attack me having 8k health and 500 armor, well, my team should use it on our favour.
On the other hand, health is always helpful imo. Yeah, %health true damage will always finish you in a certain number of autoattacks, but the more health the more you will survive other damage sources (Orianna, Lux, whatever). Again, if Vayne is getting you from 100 to 0 without punishment, then you or your team are doing something wrong.