"Would I be able to add the XP drive from the previous computer as a slave to the Vista computer then use Vista's boot program to recognize XP on the slave drive?"

No. XP will not run after the move. While it may be possible to add XP to the boot menu XP doesn't survive such moves due to many registry entries and more. There are some that have done this but you need the full XP CD with the service pack you are running and you run what is called a REPAIR INSTALL on the XP OS.

Frankly this is fairly advanced so maybe some boot manager like Acronis Disk Director and an install like you read at http://www.windowstalk.org/dual_boot_vista.htm

Bob