Essentially Vent just needs to have outbound access to the server on the port you/your friends have the vent server listening on. Put this rule in your firewall and you will be granted with the message "the server is available, press connect" and/or "synchronizing". This is where I got stuck because according to the people at vent this is the only thing you need to do - anyway - I started monitoring all of my blocked connections and found out you also have to allow port 6100 outbound - this is evidently how vent connects to everyone else you're chatting with and syncs things up.
- allow outbound access to the vent server on the port it's listening on
- allow outbound access on port 6100