Nope, I'm afraid you have a major conundrum on your hands. The only way to change the size of the partition for Windows is to delete it and start over. There are no tools that allow you to do this on the fly, and none that I've seen that can dependably and safely change partition sizes from within either OS X or any Windows OS.
You can image the Vista partition, use the Boot Camp Utility to get rid of the partition, then start over (I'd suggest 20GB or more for Vista), restoring the image when you get to that point. That requires a Windows imaging program (there are free ones around; check out the Bart PE pages
for plugin-able modules, including a DriveImage plugin).
Sorry, but I think you're stuck with this choice. For what it's worth, 8GB isn't really a decent size for XP, even a stripped-down version of it.