In Vista there is no "Logon using dial-up networking" option (Or at least I can't find it) instead the "trick" is to create a VPN connection, dial-up to your company domain, join the domain, reboot and then logon with the local user. Then dial-up to your VPN connection again and select the padlock icon, use "Switch User" (While keeping your VPN connection open!) and now logon to your domain with the new domain account user.