Having a dual (multi) boot menu is very handy if you boot to either OS on a regular basis.
But, if you boot to one most of the time, and want an option to just occasionally boot to the other, you can do it much easier and cleaner.
Just, install XP on the first partition of a second hard drive. If you need to boot to it, press F11 (may be another function key depending on your motherboard) to get to the boot selection menu and select the second hard drive as the boot device and you will boot to XP. The next time you boot, it will boot to Vista unless you select the boot menu again and select the second drive.
This is clean because each installation (on each hard drive) will have a clean MBR and clean boot file. If you some day want to get rid of XP, you just delete it! You will not have to do anything to your Vista installation.
The disadvantage is that every time you want to boot to XP, you will need to press F11 at the boot time to select the second hard drive. But, if you only do it occasionally, it should be OK.
Just an option!