As a standard NetBIOS over TCP/IP application, VisionFS uses UDP ports 137 and 138 for naming, and TCP port 139 (by default) to listen for connections.
You should configure your firewall to prevent external access through these ports. If you do this, then computers outside your firewall won't be able to set up a CIFS bridge to a computer on your network, or set up Internet workgroups with your workgroups.