"Vayne should be a tank killer"
The implication being that she should lose every trade against non-tanks. So let's look at Vayne in teamfights from that perspective:
The tank goes in, gets his CC off and muh tank-killer Vayne starts focusing him. Now, get this: The tank starts running away. Which he's quite good at
plus all the random CC that exist in teamfights. Congratulations, muh tank-killer Vayne now does nothing.
Her only hope of doing anything is to catch out a splitpushing tank since she will lose lane to the non-tank enemy bot lane. :^^^^^^^^^^^^^^)
Edit: Your reactions were worth the downvotes [slayer-pantheon-popcorn]

and turn into an everything killer.
and allows them to skip items designed to be effective against defensive builds.