Running Leopard 10.5.2 on an iMAC 24 with 2GB total RAM, Fusion 1.1.1
When I try to shut down guest to allocate more RAM to Vista,
Vista always reboots. If I try to shut down Vista from its start menu, the whole computer hangs.
Have you tried typing this into the Start menu:
shutdown -s -t 0
Be sure to save any work you care about first. If that works, you can create a shortcut on your desktop with right-click > New > Shortcut, typing the above in the location box.
If this started with applying SP1 you may have a driver conflict. Did you apply SP1 through Windows Update or direct download? Either the blue screen should identify an offending *.sys component or if you reboot, the details of unexpected shutdown message could name the driver.
If it is a driver conflict, the following Microsoft KB article may help: http://support.microsoft.com/kb/948343
I'm running Vista Ultimate x64 SP1 (VM) and Vista Enterprise x86 SP1 (Boot Camp) and both of these editions shutdown gracefully so should you be able to get your system into a good state.