Why do you want to use vista, vista is just an atempt to make a little more money before releaseing there real new OS.
Any ways to answer your question, if you don't want to completly restore the xp system then you will need to get some partition software such as norton partion magic. If you are willing to restore xp you can use the disk to create 2 partions on your drive.
I do know this - xenforo dropped the ball by not keeping the vbulletin reputation comments as a feature. The loss of the Reputation comments data when we switched to Xenforo really was the death knell for the site when it came to all the users that left. I know I missed it so much and I got way less interested in the site when that feature was gone and I run the site.