A. you can always pull your Vista drive.
B. Clone OS X to a new drive.
C. When you installed Vista, I always remove other drives so it doesn't "tag" them in any way.
D. Keep a separate drive for data, downloads, media files etc.
E. Keep your boot drive for OS X for just the OS and applications (maybe a small home account directory).
OS X makes it easy to have home account folder on another hard drive.
I almost never ever find need to reinstall OS X, I just use SuperDuper to clone the system as needed.
The actual process of erase/initialize is quick, and installing OS X takes 20 minutes or less though it depends on 3rd party applications and stuff how long it can take.
If you backup or copy/move your home folder, that can reduce the time to customize, import and have your old settings up and running.
Invest in some backup drives, internal and external, for SuperDuper (bootable), TimeMachine, and for Vista if you want to keep it backed up (or use WinClone if you don't need a real backup). 640GB WD Caviar goes for $76.