This may be totally placebo, but I noticed that using Vista inside a VMWare Fusion VM has just turned from nearly unbearable slow to actually quite fast by updating from 2.0 Beta 2 to 2.0 Final.
It may very well be that the beta versions contained additional logging and/or debug code which was keeping the VM from reaching its fullest potential.
So if you are too lazy to upgrade and still running one of the Beta versions, you should consider updating. For me at least, it really brought a nice speed-up.
gnegg 