Hi delivery1man, welcome to NST.

Make Vista's drive boot first by changing the boot sequence in the BIOS, install EasyBCD in Vista, add the entry for XP, and last of all make any changes to boot.ini (...rdisk(0)... entries changed to ...rdisk(1)... etc) needed to point to the correct disk for XP if XP doesn't boot when you select its entry on the boot menu.
It's all described in the wiki. Have a good read before you start and follow the troubleshooter link if you have any problems getting it to work.
Post back if you still can't figure it out and one of us will be around to help at most hours of the day because we're spread across at least 10 time zones.