I'd say the mix up is between he overall I/O processing channel between the CPU and the graphics card, which is significantly uplifted with PCIexpress. This buys back if nothing else, bus bandwidth and headroom.
As for using or not using the GPU. Well, this may be indirect usage. You may be driving the metal with a directx routine that shades or redimensions a pixel, surface or texture channel. If that functionality, especially the shading of video, isn't natively combined by the card/driver-support then it will be slower if the CPU has to help along the way.
VT and SE have 3D graphics card requirements and optimally on the latest hardware (with any onboard accelerated or not graphics) disabled. Good things have been said about Intel X3000 graphics, but lets assume they don't exist and that we need a good directx (why not OpenGL too.... often it follows for the lower end) card. 0.25GB of memory infers a recent GPU etc....
Even 2.5D can need a little more than a generic AGP4X 2D card, if that help this post give further clarity? Not that I'm certain that it needs to.
Vegas (apart from when using 3rd party plug-ins like BorisFX, vdub/avs and Wax) uses only 2D functions as of all versions up to and including 7. An AGP 2x card with 32MB of RAM for dual head is ample. It isn't as cutting edge in the throughput department as any NewTek products, IMHO.