Vayne and Fiora are near impossible to play against for any tank in my opinion. They both have low cooldown dashes and consistent Max HP True Damage.
There are three main ways to counter tanks as a damage carry; Percent HP Damage, True Damage, and Penetration.
Percent HP Damage is a fantastic mechanic, allowing damage carries to amp up their damage against tanks without being completely unbalanced when damaging squishier champions. Percent HP Damage can be countered by tanks if they build resistances.
True Damage is another fantastic mechanic, ignoring resistances entirely to cut down tanks that focus on building resistances. True Damage can be countered by tanks if they build Health.
Penetration is yet another fantastic mechanic, allowing all your damage of that type to ignore a portion of their resistances, increasing damage across the board. It can be countered by tanks if they build more resistances, but Percent Penetration makes that less efficient, so you need to make sure you’re still building enough damage.
But Percent Max Health True Damage cannot be countered. The only things you can do are heal or shield. Unless you’re Morde with a Spirit visage stacking heal and shield power, it’s probably not enough, and then you don’t have any damage to deal with Vayne/Fiora.
Now, if the Percent Max Health True Damage were confined to something like an ult, rather than vitals or as passive on hit damage every couple of autos, it would be an ability that you have to watch out for and play around. Instead, the damage is a consistent threat. You cannot exist near them for any significant duration because you will be summarily deleted. I’m even fine with Missing Health True Damage and Current Health True Damage, because at least those are situational. Missing Health is an execute, only useful when you’re already nearly dead. Current health drops you low so you can be dealt with, but won’t kill you.
I’m fine with mechanics and champions designed to counter tanks, but these two make locking in a tank a hard int from champ select.
It is a stupid mechanic because it is good into no matter who.