If you want XP and Vista as totally separate OSs then you would unplug the XP drive when installing Vista on the other. Otherwise the Vista boot loader and other files are copied to the primary drive with it's own boot loader as the default. EasyBCD allows you to set XP as the default OS to boot while still seeing the initial Vista boot screen.
I just set the bios boot from the other drives and disable the xp one so i don't have to open the case