Can try this and see if it helps. It for wireless however still should be able to gleam some information. Search is your friend. Hope this might help. NE one else out there that can help?
http://forums.dlink.com/index.php?topic=12254.0
Sorry but this doesn't help remotely. Not only does the thread appear to be unresolved, but it's dealing with the DIR-130, which is a router with hardware VPN support.
The DGL-4500 only has VPN pass-through, and I'm trying to figure out exactly what this means beyond creating virtual servers (port forwarding). I have a Windows XP machine setup using built-in windows VPN server. Doing a netstat on that machine shows it is listening on the PPTP port, and I am forwarding this, but I still can't connect to it remotely. I get "Error 800" in Windows 7 when I try to connect.