I originally only partitioned 70GB out of my 250GB drive (the rest was set as free space) and installed mac os on my dell... knew that I would install vista back on.
You should be able to change the size of your mac os partition without having to reinstall...haven't tried it but came across this
http://www.macworld.com/article/55274/2007...geekfactor.html.
After you free up some space on your HD to install vista, you'll have to pop in the vista dvd. It probably won't allow you to install off the bat since even though it'll allow you to create a new partition with the free space, it'll say it can't find a suitable location... at least I came across this when I tried to install vista after getting my mac os configured exactly the way I wanted (wireless working, dual core, sound, etc and didn't want to wipe it clean to install vista first).
You'll have to go to the command prompt utility off of the vista dvd and use diskpart command to set the new partition as active... reboot then vista installation will allow you to install it on the new partitition... I'd recommend after you install vista to go and download EasyBCD. Configure EasyBCD in vista to add your MacOS partition to allow you to dual boot... works great in my set up.... only complaint is that if I select MacOS to boot in, it'll start the darwin boot loader and then I have to select my MacOS partition before it loads up.