bittorrent - Curl over interface for port forwarding

I'm trying to set up OpenVPN and Deluge to seed torrents via the VPN.

I am running other services on the server, so I only want the torrent info to go over the VPN. The way I've accomplished this so far is to bind all traffic to the torrent interface via the client.

The issue comes with port forwarding. The procedure is described here: https://www.privateinternetaccess.com/forum/index.php?p=/discussion/180/port-forwarding-without-the-application-advanced-users/p1

To get a forwarded port the request needs to go via the VPN. I cannot seem to curl via the tun0 interface - either I get no connection, or the routing is messed up.

Any tips on solving this problem, or a better approach all together?