My friend and I are both on the same VPN network, our firewalls are down, and we can ping each other, but when we launch Diablo Hi Res, we don't detect each other (the hoster's game isn't visible to the other person).

If this is a matter of the game grabbing the wrong IP, can you advise me on how to prioritize the VPN connection over my ISP connection?

EDIT: We've already checked and our VPN adapters are the highest prioritized already, so I don't know what to do!

Are you running the DX version or the classic version of Diablo? Is the VPN service something like Hamachi or Game Ranger?

Game Ranger for example has issues when running anything else than classic Diablo, meaning it probably wouldn't run the DX version properly.

I'm running the HD version, not the classic version. The VPN service is OpenVPN hosted by a linux web server I rent.

Well I never used OpenVPN, but they have a nice writeup that should be helpful: http://judebert.com/progress/archives/59-Setting-Up-A-Gaming-LAN-with-OpenVPN.html

I already know how to administer and run the OpenVPN service. My question is about the technical details of the IPX wrapper that GOG is using, so that I can research why the IPX wrapper isn't working over my fully set up and working OpenVPN.

did you run ipxconfig.exe and select your OpenVPN adapter in there?

Most helpful comment so far! I will look into this later tonight and report back. Thank you.