I cannot answer regarding VPNs as I live in a country where VPNs are irrelevant for torrenting, but I can answer the port question-
If you don't have an open port for torrenting, you can only connect to other clients who do have an open port. If you have an open port, you can connect to all other clients. Either client in a P2P setting will need an open port to communicate. If neither have an open port, they cannot communicate.
It is beneficial to be able to open a port for torrenting, but keep in mind that you are essentially broadcasting your intentions with an open port (unless you use a VPN). If you live in a country where ISPs can hand over data to anyone who asks, they will use your open port against you. (unless you use a VPN).