What do Vayne and Ahri have in common...
...that leads them to be constant balance problems?
High mobility / self peel + hard CC.
Just stop and think, "Would Vayne/Ahri still be good without their hard CC?"
The answer is yes. The fact that they already have great kits for what they are supposed to do, then get a hard CC thrown on top of it makes them extremely hard to balance.
TL;DR Rework both Ahri and Vayne's E to something other than hard CC and they'll no longer constantly be complained about.