I am have set up a vpn server (openvpn) and I have a client too. I want to set it up so that a second server can only be access from the vpn server over SSH. How can I do this?
Add a firewall rule on the second server so it only accepts ssh traffic that comes from one of the addresses that the (Open)VPN server hands out.
If VPN traffic comes in on server 2 on a special interface you can also configure sshd to only listen on that.