Ok, I think I found the main point of confusion. Basically VPN needs PPPtP on port 1723 forwarded to 192.168.1.XXX but it also needs the firewall opened for GRE traffic. Where the confusion is GRE uses traffic TYPE 47, NOT port 47. GRE no more has a specific port then TCP or UDP has a port. The problem is this router doesn't allow for PPPtP passthrough on the Application Level gateway (ALG). Now that fix for that is put 192.168.1.XXX in the DMZ to bypass the firewall. However, I believe with Dlink routers, putting a IP into the DMZ makes it bypass all forwarding. And port 1723 isn't being stopped by the firewall like GRE is, but rather by NAT.
Long story short? I don't think this router will work for your purpose. Does any co-workers have this model working for the VPN?