% Health True Damage

There IS counterplay, just not counterbuild. Look, vayne has to put 3 stacks on same target right? So, force her switch targets and stacks get lost. Fiora need to attack from direction of the mark. So, you know where she will strike from, if you're next to wall she can't hit it, it's basicly her only DMG source, and you can always be ready to fight back.

And, it's not really strong unless you're tank and it's late game. In that case, don't fight fiora 1vs1 (she's suppose to be strong at that, remember), and if you're in Vayne's range (which is pretty close) attack her. Unless you're immobile tanky champ, which she was made to counter

Only a problem if in high numbers aka fiora

maybe you should try using shields or something that lowers attack speed or slows them down?