I'm not a Vayne player, but what do you guys think of a change like:

Vayne's silver bolt now stack up to 4, but will not be procced automatically on the 4th auto attack. Additionally, an auto attack empowered by Vayne's Tumble will proc all the stacks on the target for additional damage dependending on the amount of Silver Bolts stacks on the target. In addition, each Silver Bolts stack that is procced will reduce Tumble's cooldown by 25%.

This will give enemies the chance, to a certain extent, to counterplay the incoming damage (mainly by giving them a bit more time to react), while the player on Vayne will be able to outplay his enemies by timing her abilities correctly, while allowing some versatility by proccing the Silver Bolt's stacks earlier.

The numbers I mentioned are the one that to me make most sense, but of course I may be mistaken. I just thought that Vayne is quite a difficult champion to balance in her current state: like Azir, Kallista and others, she will most likely either be underperforming or melt everyone faces in an ult.