AFAIK you cant use Vista as a secondary OS in terms of boot selection. Vista has it's own boot.ini file, so no matter what order the OS's are installed, Vista's boot.ini will appear first on the list. For instance, in your case every time you boot up the PC Vista will be the primary option, and you need to manually scroll down to the second option which is something like "boot from an older version", and then choose XP.
But you can dual-boot both OS's. If you only have one hard drive then you will need to create a new partition, and you can do so with a program such as Partition Magic.