I think it's important at this stage to decide if supporting OpenGL in the future is something you actually want to do.
A lot of people start out with the idea of supporting OpenGL (or DirectX if they start out with OpenGL) at a later point in time in their rendering engines; this results in them trying to write an abstraction layer on top of their original graphics API which doesn't really do the underlying API any justice.
Designing a proper wrapper exposing the functionalities of both APIs to their fullest extent is HARD. It's going to require a complete understanding of both APIs, their designs and their quirks. Add to this that you'll have to design a system which will unify these sometimes completely different beasts into a single API without introducing too much overhead and without sacrificing functionality.
If you really do want to support both you could always check out some third party utility libraries for doing vector math and such, I'm sure there'll be plenty of those out there.
If support for multiple rendering APIs isn't an absolute must I just wouldn't worry about it and work with the tools provided by SharpDX; no need to put a lot of effort into something which eventually won't have that much of an impact on your end result (ie. your game)