it's a little bit complicated, but it can be done
First you have to find a windows Vista compatible driver for your card. If it's a desktop GPU just go on the amd-ati website, if it's a laptop card you have to go on the PC manifacturer website. If the manifacturer of your laptop doesn't support Vista just find another one that produce a model with the same screen and the same video card.
When you have the correct driver go here: http://www.hardwareheaven.com/modtool.php
and download Mobility Modder Software (you have to run it on a windows XP, Vista or 7 machine, it doesn't work on win8). It will modify your driver and make it work under win8!!!