What ports do I need to open to permit VPN traffic? (by 2 users)

For PPTP VPN connections, you need to open TCP port 1723 (for PPTP tunnel maintenance traffic). PPTP also uses IP port 47 for tunneling data. Port 47 is designed for "General Routing Encapsulation" or GRE packets.
For L2TP VPN connections, you need to open UDP port 500 for Internet Key Exchange (IKE) traffic and UDP port 1701 for L2TP traffic.