Hi there. If you divided from the XP partition there's a chance you may have damaged the system files on that partition. Although I've never used the software you're talking about, setting XP's partition as "hidden" may also have contributed.
You do have disks for both operating systems, correct? If so, insert the Windows XP disk and try a
Windows XP Repair Installation. Do the same with Vista. I've never experienced Vista's upgrade process but I imagine the steps are similar.