How do you know it’s a bad idea without knowing what the goal is?

Assuming your vpn provides a stable remote IP, your client connection needs to use that. Try “whatsmyip” or similar over the vpn. The remote address almost certainly won’t appear in the local output of ip a.

Locally, listen on the “this host”, 0.0.0.0.

You may need to check your firewall locally.

You don’t need to run your http service to troubleshoot - simple tools like netcat can listen for incoming requests - nc -l 0.0.0.0 8000 or what-have-you.

Finally: you might want to look at using a shell host as the client rather than targeting your vpn ip from your local host, just to take hairpin connections out of consideration when troubleshooting.