A couple of tips for you for when you get your partitions sorted:
Just run the Vista install .exe from in XP, select custom install then select the destination partition
Get EasyBCD to edit your boot order (Boot.ini in XP, BCD in Vista) and other little extras, run it from in Vista.
When you decide to remove Vista, use EasyBCD to return your boot order to XP being first BEFORE you remove Vista as EasyBCD won't run under XP and if you remove Vista without correcting your boot order there are only two ways to fix it, reinstall Vista and EasyBCD or format your XP install to repair your boot.ini.
Easiest way to remove Vista, delete its partition then recreate it for further use.
Well that was how it was last time I dual booted them.
Cheers