I dunno what exactly your problem can be, but have you tried any of the other settings VNC uses for screen refresh? With my setup, certain scrollable windows wont update when I scroll unless I move the mouse over it. The window itself isn't sending a screen refresh signal, so VNC only refreshes whats directly under the mouse. Other programs work fine though, and refresh the entire window contents like it sould when I scroll.
Also, have you tried disabling ZA or setting it to Allow All just to see if it really is ZA causing trouble? If you can eliminate ZA as the culprit, then you can play around with VNC's settings. But if ZA turns out to be the culprit, and theres nothing you can do to fix it, try a different software firewall. Sygate Personal Firewall is my favorite, and it works fine with VNC. I also have it configured to allow connections to VNC on only 1 port, and from only my ip address. Extra security never hurts.