SSH server can't be connected to when VPN is turned on

I recently found that when my workstation connects through a VPN connection then its SSH server can't be connected to from a remote site. I am sure it's a routing problem because the the VPN client changes the default gateway to its peer (VPN server) of the ppp connection.

Is there a solution to make SSH server and VPN client both happy?

When you use a vpn generally the vpn network takes over your entire interface such that you are only routable from somewhere on the vpn network not the internet at large. Most people solve this problem by running a vm (virtualbox etc) and connecting to the vpn in that virtual machine so that it doesn't completely hose the main connection on the actual machine.

Before adventuring through the network setups, check if the ssh server in question listens on the vpn interface. Maybe it is bound to a specific interface on your server.

Example netstat -a output:

 Proto Recv-Q Send-Q Local Address    Foreign Address   State      
 tcp        0      0 *:ssh            *:*               LISTEN      

The ssh server in this example listens on all interfaces (indicated by the asterisk in *:ssh. If on your system there is a host address instead, the ssh server is bound to specific interfaces.

Edit /etc/ssh/sshd_config and set ListenAddress 0.0.0.0 to adjust this, if neccessary.

If sshd already listens to the correct interfaces, feel free to enter the routing dungeon :-)