VNC sets up a desktop of it's own, in which the Xserver on your workstation channels everything from the VNC desktop to a virtual monitor. What you must understand is that the vncserver runs as a session in it's own right. It will mirror all the settings of the user who starts it, and all the settings from that users home directory. A way to accomplish what you are talking about is to create a new user, however that new user will have to have access to your home directory for obvious reasons.
A suggestion for you is to look at nx technology, take a look:
http://freenx.berlios.de/info.php
This may allow you to mirror your own machine more securely and with more responsiveness than VNC. I dunno about different settings tho!