Vizard simply wraps the DirectSound API. So Vizard does not do any additional proccessing.
So the nature of the room 3D model does not affect the 3D sound that occurs in it. Only the sound-source's/listener's position and orientation factor into the 3D sound algorithm.
From the DirectSound API documentation, the 'HRTF_Full' or 'HRTF_Lite' options allow you to trade the quality of the 3D sound effect with CPU cost.
http://msdn.microsoft.com/en-us/libr...68(VS.85).aspx