Wireless Authentication

I have a Dell laptop that is using netscreen remote to authenticate to the user’s VPN. When logging into the laptop using their cached domain logon, using a wired connection authenticates and the VPN can be accessed. However, when wireless only is used, a logon prompt comes up asking for credentials and when entered, the domain controller cannot be found. If I login to a local laptop profile, then connect VPN via wireless, the credentials can be entered and the VPN is authenticated. Help!