Win 8 VNC connect via ssh, error connect failed...

Getting a Win 8 , VNC viewer to connect via ssh. I establish a ssh connection with a tunnel. When the VNC connection starts it is refused and on the mac system console I see "SSHD error, connect it my_server.net port 5900 failed". On the mac I have ssh and vnc enabled in sharing settings. A normal non-tunneled VNC connection works when I open the firewall (Airport Extreme) for vnc connections. Also do have a password for connecting to the vmc server. When testing the tunnel I have only port 22 open.

It appears the SSH and tunnel are working correctly. (using putty on the Win8 side and log shows a tunnel established and we get logged in in a terminal window, plus the console log show we are knocking on the door) The VNC viewer doesn't have a way to enter a password on connection so I assume it will prompt for one when a connection is successfully established.

Any suggestions as to why we can't establish a connection?