I had the same problem using RunebergVRPlugin. I wanted to derive a pawn class from RunebergVR_Pawn and there were compile errors. I had tried many things including adding paths in the project settings, but nothing worked. Finally I found the following page on the wiki and at first it did not solve my problem. So I undid all my experiments, removed paths from project settings and did only what the wiki page said and finally my project compiled.
FYI: Even though everything compiles, there are errors showing on the VS Error List window and squiggly lines are showing from missing intellisense. There are no errors in the Build Output though, so seems better than not compiling at all.