If it were an upgrade it would have installed to the same partition as your previous OS and moved XP's files into a folder called Windows.OLD in C:\ for your review and eventual deletion. Since you're speaking of removing the XP partition it sounds like you actually performed a clean, parallel installation of Vista along side XP. If that's the case then removing the XP partition will indeed cause Vista to become unbootable since all boot information is stored on that XP partition because XP was installed first. Thus, after reformatting the XP partition you'd have to boot to the Vista DVD and run Startup Repair, after clicking "repair my computer" on the second screen, to fix the problem.

Before doing this, though, you should verify that:
1.) XP and Vista are installed to separate partitions (browse the drives listed in My Computer)
2.) You have a retail Vista DVD
3.) You've backed up all of your personal files on the partition you're about to delete.

John