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.

You could unload the screensharing service (and thus remove the need to install software):

sudo launchctl unload /System/Library/LaunchDaemons/com.apple.screensharing.plist

and start it again by:

sudo launchctl load /System/Library/LaunchDaemons/com.apple.screensharing.plist

Alternatively, you could create an alias (a custom command) by adding a line into your .bashrc file (an explanation of the difference between .bashrc and .bash_profile on OSX is here: What is the difference between .bash_profile and .bashrc?)

The alias line may look like:

alias myvnc='vncviewer server:port' (whatever the executable and paramter required to start the program)

Save .bashrc and exit. Then type source ~/.bashrc in the terminal to reload the file to test your new alias. In this case, typing myvnc will trigger the command you have entered between the two quotes.

This can be used for anything to start things quickly via terminal with your own preferences.