Technically yes but no, at least not easily.
The only way you could do it would be to create two partitions from the partition that BootCamp made. Depending on the size of this partition you could be out of luck because Vista needs a minimum of 20Gb to install.
Install XP on one of the partitions created by BootCamp and Vista on the other. Now comes the tricky part. Vista will override the bootmenu and will not give you a menu to run either or. You will need to alter the boot.ini and create an entry for WinXP.
From then on it should be as easy as selecting which OS you want to boot from. You'll have to use Google to find out how to do this because I don't want to further your evil plan anymore.
