If you want to use boot priority that most motherboards offer and you can access by pressing F8 or F12 or one of the function keys during post depending of the motherboaqrd, you should disconnect the XP drive while you install Vista.
Then, when you start the computer, you need to access the boot priority if you want to boot to the OS that is not the default.
You can change the default by editing the BIOS setting.
If you want to get a menu automatically every time you start the computer, leave the XP drive attached while you install Vista.
The disadvantage of this approach is that if some day you decide to remove Vista, or if you just disconnect the Vista drive, you will lose the ability to boot to XP until you fix the boot manager.