Remove the existing connection on the Vista machine and set it up again :-
1. Go to Control Panel > Network and Sharing Center > Manage Wireless Networks in the left pane and right-click the one you want to remove.
2. To connect go to Control Panel > Network and Sharing Center > Set up a Connection or Network in the left pane > Manually Connect > Next then key in the SSID, select WEP/WPA and type in the key. You may also want to tick ‘Start this….‘ and ‘Connect even……‘.