Okay first things first is to recreate the Vista entry in the bootloader.
Boot from your Vista install disk, select the language setting you want and then on the next screen pick to repair your install, let it fix the install automatically. After the reboot do the exact same thing to see if your version of Vista is showing up in the repair screen., choose to repair the boot loader.
It wont actually repair the bootloader but when you are in Windows XP grab a copy of VistaBootPro
VistaBootPRO - Go PRO with Microsoft Windows Vista Boot Manager there are other programs that can do the same thing but I am not familiar with them, so I will use this one.
Create the back-up if you want and then go to the
Manage OS Entries screen, you should only see Vista on here (weird I know), so check the
Add New OS Entry box, give the OS a name and then point it at the drive where XP is installed and pick
Legacy as the OS type. Hit Apply Updates.
Then go to the
System Bootloader screen, choose
Windows Vista Bootloader and
All Drives as the two options, then hit
Install Bootloader.
Now when you reboot you should have the OS selection menu. This worked for me when I wanted to do the same thing.