Someone remind me why this is ok

s6 complaining about vayne? really? she's hardly a mid tier adc right now

Probably because that's one of Vayne's only strengths? Why not cry about Jhin doing 3k damage with his ultimate while suddenly getting a burst of 100+ ms every 4th shot or Sivir autoing creep waves to hit the backline for 500 damage crits? Maybe even the Ezreal who has a blink on a 3 second cooldown at this stage of the game.

To rack up that much True Damage before falling, you have to be a Rammus or another high HP + high Armor tank. Killing the ADC is not your job, while Vayne's job is killing champions like you. Simple.