fusion doesn't support aero
I was having the same issue with Aero... but drivers?
Are you sure the drivers are completely gone? You could try running the "Windows Experience Index Updater"... that's what I did to get Aero working again... I eventually just uninstalled VMware though... my engineering apps are too graphics intense to use if Vista can't recognize my graphics card.
After installing VMWare Fusion on MacBook Core 2 Duo, I booted directly in Vista via BootCamp and my video drivers were gone. Cannot do more than 1024x728.
I believe I have had this happen recently as well, a reinstall of the bootcamp drivers on on the Windows partition gave me aero back.
It does, until you next run Fusion...