請問你的Nortel VPN是哪個版本的呢？
據了解是要6.07版才能相容於Vista。
原廠網址如下：
http://support.nortel.com/go/main.jsp?cscat=SOFTWARE&resetFilter=1&poid=10621

網路上找到的，參考看看
· Enable the NAT Traversal on the VPN Contivity Server -> Services-->IPsec. Under IPsec, & UDP encapsulation port, put the port there as 10001. Click on OK. Please note that this requires admin previleges on the server which I have. J I have not created another group for vista users since only few users..but use your own discretion.

· Download the latest version of the VPN Client from the Nortel website. Current version is V06.07.027. You got to be a register member of this site. http://support.nortel.com/go/main.jsp?cscat=SOFTWARE&resetFilter=1&poid=10621

· Uninstall any older version and install this latest client and reboot the machine. Configure the client with username/password/destination & connect…Vola!!

Previous issues encountered:
· Enable the NAT Traversal on the server..
· Unable to connect to the server. If there is a firewall configured enable outbound traffic to UDP port 100001.

來源網址
http://forums.microsoft.com/TechNet/ShowPost.aspx?PostID=2066914&SiteID=17