Yeah. Install XP. But after you do you will need to get EasyBCD. From there you will have to go to the XP Boot.ini. To do this go Start>Run>msconfig>boot.ini tab. From there write down your boot info.
Then use the Vista disc. Get the BCD boot manager back. Then boot into Vista. Get EasyBCD loaded. From there add XP to the Vista Boot loader and you are all set.
EasyBCD can work from within XP but that process is much more tricky. The way i just stated is the easiest.