How do I create a vpn over another vpn?

From my testing NetworkManager (using nmcli) only supports one VPN connection at a time. You will need to write a script that manually invokes openvpn and place it in /etc/NetworkManager/dispatcher.d/. The script will need to query networkmanager to see if your other VPN connection has been made.

I wrote a script that automatically starts my VPN connection under certain conditions, maybe you can use it as a starting place.

Also, keep a close watch on your routing table. If you start a VPN while connected to another one, it could redirect traffic from the first VPN, disrupting both VPN connections.