Simplest solution to balance vayne
What makes Vayne strong? Her true damage. What you can do to counter Vayne's true damage? Absolutely nothing. Why can't you do anything to counter Vayne's true damage? Because it is Max Health true damage. What can you do to balance Vayne? Make it Current Health true damage, meaning that you will be able to counter her true damage building health something you can not do right now because she will just burst you faster.
Thoughts?