What software are you using to partition your hard drive? Are you using an upgrade copy of Vista or a Full Copy?
I would recommend following the information found here:
http://en.wikipedia.org/wiki/Ntfsresize if you are looking to resize your current HD into two partitions without losing the XP.
That said, I haven't tried dual booting XP and Vista (just SLED 10 and Vista). I don't know how Vista handles dual booting. With Linux, I use the Grub boot loader to handle the OS selection.
Can anyone else answer questions about the boot manager in Vista?