How to change the default VNC client?
Try the RCDefaultApp preference pane at http://www.rubicode.com/Software/RCDefaultApp/. It lets you choose the application to be launched for all sorts of situations.

Install it, open it (it becomes the "Default Apps" preference pane), and then choose the "URLs" tab:



Scroll down to the "vnc" prefix, and then choose what you like from the "Default Application" popup.