User blog comment:StrategicMagic/Vayne - Carry? Yes. AD? No./@comment-5266525-20120819110651

Um... you can't build anything at all to counter Vayne's true damage as its like the only true damage in the game that does a percentage of max health, so it can't be countered by just stacking health, unlike all other sources of true damage.

Also, you just don't build Madreds on AD carries because you don't itemise for magic penetration. Against targets whom the health-shread really will make a difference they almost always stack enough MR to make the damage irrelevant. Against targets with low enough MR for it to deal almost full damage you'll just end up doing more damage with autoattack anyway.