If Vista is installed on a second primaty(not likely) you would have to see that moved to the beginning of the drive once the XP primary was removed. When booting up with the Vista installation disk the options for repair tools is seen in the bottom of the screen while the "install now" button is seen in the top half.
The automatic fix startup problems tool is seen along with the few others on the next screen once choosing the repair tools option. That will see the boot information rewritten as well as recopy boot files needed to the Vista primary once used.
While booted with the VIsta dvd you can cancel the installation and still use the partitioning tools there. But at times the MS tool always seems to leave 1-8mb of unallocated drive space later noticed in the Disk Management tool with both Vista and XP drive tools alike.