From the Microsoft EULA for VISTA :
15. REASSIGN TO ANOTHER DEVICE.
a. Software Other than Windows Anytime Upgrade. You may uninstall the software and
install it on another device for your use. You may not do so to share this license between
So Technically you can as long as you are uninstalling it from the one system and then installing on the other system each time and not using both at any time it is installed on the other !!