I am sure that there are no managed 3D libraries supporting both XNA and Mono (i.e. Open GL). I would suggest that it would be best to write your own minimal abstraction layer on top of Xna (which just does what you need) and then implement it in Mono + Open GL.
Either that, or you could start and implement the Xna 3D API in MonoGame or ExEn. I recall a thread on the MonoGame forums about someone who has a very simple initial implementation of VertexBuffer. Obviously, this requires a pretty decent knowledge of OpenGL.
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