Zend certified PHP/Magento developer

Debian how to route traffic from/to a web app through eth0 instead of tun0 (VPN)

I have a web app which calls some public APIs which block VPN connection. I would like to route all traffic coming/going to the web app through eth0 and not through the default tun0 which is the interface used when the openvpn client is on.

How can I do this?