Make a file, $HOME/.desktop, and put the string "KDE" or "GNOME" in it depending on the user's preferred X manager.
For information on this procedure and more on how X works see:
See the section 34, "How X works" under the header "How to set up X managers for different users". It explains how to set it up for various users and how to set the system settings.
Also the Linux User's guide at:
Item 42 describes the installation and use of VNC with X.
Let me know if this helps and how much.