I think I have an easier solution, at least in 10.8.2. I, too, had a VPN configuration that would not go away; the - button was grayed out. Here's what I did, though only step 2 is mandatory.
- In Terminal: networksetup -listallnetworkservices
- In Terminal: networksetup -removenetworkservice networkservice
- In Terminal: networksetup -listallnetworkservices
The first step shows you the names of your networkservices, like "old." The second step removes the network service. Be sure to spell the one you want to remove exactly as it's listed in the results of the first step. The third step confirms it is gone. Only Terminal is required, no XCode. The .plist will automatically update; no need to copy or delete. The remove command bypasses the GUI and removes the service whether or not the - button is grayed out.
Hope this helps. If you run man networkservices you will see many other Terminal commands that maniupulate items in the Network preference pane.