it doesn't automatically add my VNC Windows service, so I can't add any firewall exception. By default, the very last Avast firewall network rule will block all incoming TCP/UCP traffic and that can't be changed (AFAIK?) and for some reason even if I add a custom rule to specifically allow the port the VNC uses and place it "above" this public TCP/UCP IN block it continues to block using the Public TCP/UCP In block rule. I also tried adding the exe files of the VNC application as an exception but that didn't work either.
If I turn off the Avast Firewall, the VNC works ...
UPDATE:
Well it does work after fooling around with different options. I had to use a custom rule to allow the port and move it above ALL other rules. It would be nice to be able to add an application manually for those apps that don't appear in the list.