What are the rendering constraints of vive?  Triangle count?  Shaders?  Texture resolution?  Post processing?

It really depends a lot on your target and hardware and what you're doing in your game/engine. There's no hard limit on something like triangle count, you have to look at things holistically since your overall performance is going to be the sum of many factors.

Your only real hard constraint that I could quantify is that your game needs to run 90 fps consistently. For a lot of games/engines the limiting factor there is on the CPU side of things, and has little to do with your GPU performance. If it's your own engine that you're working with then I would recommend trying to architect things in a way that prioritize consistent performance over best cast low performance. Having a few frames that go over 11ms due to poor worst-case algorithm behavior can have a really negative effect on the user experience.

What are some rough numbers for extremely high end computers using unreal engine 4?