if the monitor displays properly, you should simply just need to adjust the resolution to something higher to get it to work. had a problem when someone set up a windows 7 install (quite poorly i might add) and had the screen resolution set way too high for the display that it was going to work with primarily, changed the monitor to a larger one, and pulled down the display resolution, and hooked up the old monitor and adjusted one more time and voila, it worked (also the monitor was a OLD compaq monitor...old like rocks)
-are the xp and vista machines running at two different resolutions?
as was said earlier, this should be plug and play, but if the display adapter is set to a too low or too high resolution for the display, it just will not show anything