You can set up a dual boot between the two versions if you have the drive available. I think that would be the only option before ending up needing a replacement for you.
In fact I can point at an excellent reference for seeing XP installed after Vista in order to see a working dual boot of the two versions.
Installing XP After Vista - NeoSmart Technologies Wiki
Of course then you need the free BCD editing tool found at
Download EasyBCD 1.7.2 - NeoSmart Technologies
First you need to see how much free drive space is available for first shrinking the current primary partition and then creating the second for XP to be installed on. The first two steps before proceeding advised are:
1)backup all important files
2)select a good 3rd party drive partitioning tool rather then trying to use the Disk Management for shrinking the Vista primary while booted in Windows.
Besides retail softwares like Norton's Partition Magic, Paragon, or another there's also the free live cd version of the Gnome Partition Editor known as GParted live. The latest releases of the free drive tool are seen at
SourceForge.net: GParted: Files
For the documentation on use,
GPARTED DOCUMENTATION - GENERALITIES