make sure the XP SATA drive is the boot drive in the bios.
Boot to XP desktop and insert the Vista disk.
Select new a new installation for Vista and NOT an upgrade.
It will now load files, reboot and install Vista on the drive you pick.
It will put a boot loader on XP and Vista will be the default operating system and it will give you 30 seconds to either select the " an older version of windows " (XP) or load the default (VISTA).