Have you checked to make sure that there are no other VNC Servers running on that machine, and what ports they may be running on? It is possible that you could have multiple VNC Servers running on the machine using different ports.
Apple Screen Sharing is itself a VNC server, so it may be using that port.
However, you may be running into an additional problem, which is in Mountain Lion. In order to fix this, you will need to modify a preference in Vine Server. In Vine Server, go to the Advanced tab in Preferences. For Command Line Arguments, put -ipv4, and then restart the server.
Hope this helps! Happy testing.