If you want another container to access the passthroughvpn container, you must use the internal docker IP.
Next to the container, you will see something like "172.17.0.7:25570/TCP <----> 192.168.0.240:25570" in this case 172.17.0.7 is the internal IP of the container, so if you want another container to be able to access the container via the passthroughvpn container, you need to enter, in this case, 172.17.0.7 (with the correct port).
Accessing the passthroughvpn container outside of you LAN is possible via port forwards.
You could either forward the ports of your LAN (to access it via your own home IP), or, if your provider supports it, forward ports at the VPN Provider's side. Port-forwarding in both cases should be pretty straight forward.
I hope it's clear, if not, let me know