The only nerf I think Vayne needs..
Is to revert her 4.13 buffs.
For those who don't know what buff I'm talking about:
Final Hour: Bonus attack damage increased to 30 / 50 / 70 from 25 / 40 / 55.
That would help to solve her power of killing both squishies and tanks alike, giving her more of that tank-busting power.