vnc wasn't made to do that, on *nix's. What you need is x11vnc. I don't claim to know how to because I've never needed it but one of my friends have. Please have a look at these links and especially last one at " How to use x11vnc"
General info
http://en.wikipedia.org/wiki/X11vncThe howto:
http://www.karlrunge.com/x11vnc/