I've run a couple of vent servers, I'll find how I configured the port it ran off. Just checked, I assume you're running the free downloaded server client, right? Well, you can't configure the port it runs off:
Public version.
The Public version of the server is hard coded to 8 slots per instance, one instance per machine and port 3784.
To be able to configure the port it runs off you'd need the pro version, which is rarely licensed and only to big vendors, unlucky :(