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.
That makes sense. If the same textures would be used for the second eye, then it shouldn't need to load any more. Having another frame buffer for an extra mere 2MP frame (assuming it's 1080p) should only need a few MB more VRAM.