What do i buld against fiora/vayne

You can't build against % max health true damage, its designed that way.

Against Vayne, who can only apply her true damage after 3 consecutive auto attacks, there are some options. Build to lower her attack speed and don't be afraid to build more health. Sure, the % max health true damage means she still kills you in the same total number of attacks; but that is only true if she was doing % max health every auto. She isn't. So increasing your armor and your health pool will give you more health for her physical damage attacks that are hitting. I won't go into detail into anti-vayne strategy, but know that she shouldn't be 1v1'd. However, her very short range leaves her very open to combination take downs so despite her scary true damage, she can be handled pretty easily.

Fiora is another story. She procs on a single ability use, so you can't really react to it, and can build tanky and has invulnerability to boot. She is getting nerfed this patch, which will help, and she doesn't do nearly the same amount of true damage that Vayne does (especially not post nerf using the same build she has been). If she wants to achieve higher levels of true damage, its going to come at the cost of her tankiness, and the same rules that apply to vayne will apply to Fio, just bait the W first. Bottom line, Fiora is too strong at the moment and there isn't a lot one can do vs her, but that should be changing soon.

Above all, prioritize health and and armor still, because these champions are doing more than just true damage. Since you can't mitigate the true damage, mitigate what you can. About the only nice thing about % max health true damage is it always does the same relative amount of your health; so it can sorta be "ignored."

Zhonyas. And Kayle. And GA.

What do you build against magic? What o you build against crit? What do you build against tanks?

Damage.

This is more for Vayne than it is Fiora, but being a threat to her is far more effective than trying to absorb her silver bolts.