I refuse to use it on mine, but I feel like an easy fix would be to move volatile reversal to a higher void knight level past the threshold, such that only VKs can get it.
We talked about doing that and while I can't make long term guarantees, it's not planned to be done right now. It's just such a fun skill for Paladins and Forge Guards too.
I think the better way for now is to make VK's just the best at using it. Work bonuses into other VK skills, introduce more synergies with void abilities, or maybe put things in the locked section of the VK passive tree to help with it.
I think this delivers on it being a VK skill but gives that light multiclassing feel too.