The ability says she does 8% of maximum health, meaning, no matter how much hp i have, ill still die the same as supposed to building pure armor and maybe reducing a lot of vayne's auto dmg.
The auto's that dont deal true damage ofc
Health still helps against the rest of Vayne's damage, but it's not ideal against Vayne's Silver Bolts.
Armor is a lot better because it reduces the rest of Vayne's damage output. Buying Warden's Mail and its upgrades allow you to also reduce Vayne's damage output by slowing her attack speed, reducing the number of times she can auto attack you and, therefore, reducing her damage greatly.
You cannot escape Vayne. The only way to beat her is to lock her down and kill her or shut her down hard early.
no health is terrible, buy randuin, forzen heart, and play malphite, with a nasus/darius/anivia/nunu, and exhaust her.
Her 1.8attack speed will be lowered to 0.12attack speed, it's enough to not focus her and focus the tank instead because they deal absurd damage. focus amumu is the best strategy because he can kill tank in 10sec with his abilities.
You want to get a frozen heart and randuins so she AS slows herself to death then maybe a nasus or someone else like Nunu and just unload AS slows on her and Exhaust her. Or just blow her up. Both work.
Key to Vayne. Armor and AS slows (armor for AA damage, AS slows for reduced # true damage procs). You preferabbly build regen from there like a warmogs and spirit visage to heal off the chunks.
92% of 3000 is still more than 92% of 2000. It's not as if she only does true damage, or even mostly true damage.
Sure health helps a litle less than against other champions, but there is still a point where buying health is better than buying even more armor.
That said shields and healing are the best agaisnt her, because they give you more life without increasing damage.
