Windows Vista is more than likely on the drive tagged (System) as per Disk Management. You will need to first of all backup your Windows 7 BCD (use DualBootPRO) and place the backup on your Windows 7 partition where you can access it later. It may be a good idea to create a folder on your Windows 7 partition called say "Backup". Also, copy the file bootmgr and the folder Boot (plus contents) to the Backup folder as you will need them later...these are your Windows 7 (and Vista) boot system files.
Next, from your XP installation CD format the Vista partition and install XP there. You will boot to XP after installation without any option to boot to Windows 7. From XP (after you have installed all your hardware drivers), copy your Windows 7 boot system files to the XP partition which should now be the one tagged (System) as per Disk Management) and restore your BCD using DualBootPRO. You will need to delete the Vista entry and to add a legacy entry for Windows XP allocating the drive letter it is on (probably C as seen from XP).
Reboot and this should give you a dual boot menu to be able to boot to either XP or W7.
Check out these guides for more info BEFORE you get started so you have an idea what you are in for:install-windows-xp-in-dual-boot-with-pre-installed-windows-7-t112568.html reformatted-and-reinstalled-xp-lost-boot-to-windows-7-t114759.html
NOTE that you will not have a separate System Reserved partition as mentioned in the guides because you installed W 7 after Vista which was already on the System partition.
NB: If you have neither XP or Windows 7 installed yet, first install Windows XP as directed above and then install Windows 7 after XP. That is the correct install order for Windows operating systems (older first followed by newer) and a dual boot menu will be auto created. In this case you need not backup the files I mentioned earlier.
"The great majority of mankind are satisfied with appearances, as though they were realities, and are often more influenced by the things that seem than by those that are." - Niccolo Machiavelli