I'd like a real answer to this myself. I tried to test it within Skyrim, but it was hard to get a clear picture. As I moved around, the memory usage was going up and down.
From what I can tell, 3D Vision (should be the same as HD3D in this case), used a tiny bit more VRAM, but not a lot. It probably used more VRAM for two sets of frame buffers, but was able to share texture info for each eye.