So in the same way that Varus needs to use an ability to get his percent health damage off, what if as a way to make vayne healthier, she had to tumble in order to proc her true damage. Of course then she'd need buffs elsewhere but is it a start. She would sacrifice her mobility in having to q to get the damage off.
Top-tier Vayne players already play her exactly like this to get more burst off. They intentionally hold Q until it's the 3rd shot every time. All this will do is make Bronze/Silver/Gold/Plat Vaynes even more trash than they already are.
As for Varus, his is an on-hit mechanic that has always done a small portion of magic damage on every hit. Popping it with Q or E, and being able to use his W to increase his Q damage, are easier to pull off because of his higher range. It doesn't make him more difficult, nor does it interrupt his play pattern, as his W+Q combo is meant to be a finisher for when opponents get away with no hp. In my first game on Varus pretty much ever, I was able to snipe low-health enemies through fog of war and get stupid last hits that I shouldn't have gotten by predicting where they would go. This doesn't interrupt the play pattern during combat, though.
