Solution to balance Vayne
Make her Silver Bolts ability into an active ability. Passively, she gains flat true damage every third basic attack, but when she activates it she gains the % health portion for a short duration, but loses the flat until it the active duration is finished and it comes off cooldown. This makes her choose between busting tanks, or melting squishes. I don't know the numbers, but i think this could a good step towards balancing her. Thoughts?