For future reference Michael,
If you install Vista after XP, Vista knows about XP so it will dual boot automatically, and add XP to Vista's boot loader.
If you install XP after Vista, XP doesn't know about Vista (it hadn't been written when the XP bootloader was coded) so it puts XP's bootloader in place on top of Vista's, and you can't see Vista
But it is still there and OK.
You just need to boot the Vista DVD again and select "repair" and Vista will reinstate its bootloader as if you'd installed it after XP, with a full dual-boot capability.
You didn't need a full reinstall of Vista, just a repair of the boot loader, which is an automatic feature.