Programs like Airwave and Reaper can already be used to run Windows .DLL's on Linux without modification - or compilation. I've been using a few Windows-only VST Synth plugins on my Linux DAW for years ...
VSTs are given the raw HWND to do their drawing, so I think you'd need a good chunk of WINE too to do their work.
I think VSTs pass native resources to the plugin. However, there are VSTs without GUIs, but this says nothing about what other APIs a VST may use.
I imagine there might be some VSTs that don't use a GUI that can run.