Please allow vnc traffic over port 80.
This is probably not as simple as just configuring vnc traffic to use that port (though that might be a starting experiment).
The viewer would probably have to send valid GET requests via the proxy to the server to enable a 'vnc over http' protocol to return the view data successfully via the proxy.
It has the advantage of being even simpler (to use, maybe not simpler to program

Cheers
Dale