Under Windows XP and older operating systems videomemory manager explicitly shared it between all applications running in the system. In other words, if you have 256MB display adapter, start any 3D application that eats 64MB of videomemory then try to start one more 3D application, it will see and will be able to use only 192MB (256-64) of available videomemory. Such videomemory management approach put restrictions on starting many 3D applications simultaneously, however it allowed to implement videomemory usage monitoring easily because any application could see how many videomemory is currently available and how many videomemory is eaten by other applications currently running in the system.
Vista introduced completely different virtualized videomemory management approach. Videomemory virtualization means that videomemory consumed by 3D application is now absolutely invisible to other 3D applications running in the system. Clearer way, now each 3D application always receives personal full virtualized videomemory, even if there are other 3D applications in the system and even if actual amount of free videomemory is much lower.
Such videomemory management allows starting unlimited amount of 3D applications simultaneously and I guess that 3D Aero interface was one of the main reason of introducing it. However, such videomemory management makes it impossible to implement videomemory usage monitoring via standard approaches working on any hardware, M$ cutted all ways of doing that. Probably it will be possible to invoke undocumented functions of ATI/NVIDIA drivers directly to retrieve this info in future.