Nope! You'll have to do a full fresh install of XP to go back.
If you have room to spare, I'd just resize your paritions and make room for the Vista install to be seperate. That way the only thing you need to do is restore XP's boot loader should you decide that Vista is ass.
I have a question about Medicat if anyone can answer.. My windows install is corrupt and wont boot. Does Medicat allow you to run DISM online or do an inlace upgrade? Trying to do some sort of repair install or recovery without a fresh installation