Running Mythfrontend over VNC is possible and can be useful in situations where you need remote access to a function which is only available in Mythfrontend. This will not allow you to watch video over the VNC connection, but could be useful for running MythArchive remotely or tweaking settings on a MythTV box which doesn't have a readily accessible keyboard. Detailed instructions are not provided here on setting up VNC, but they can be found in several other places on the internet.
Once you have vncserver running on the backend and you have connected to the VNC server with vncviewer. You can launch MythFrontend by typing the following command in a terminal:
mythfrontend -O ThemePainter=qt
ThemePainter=qt statement is necessary because MythTV .20 and later defaults to use OpenGL menus and VNC doesn't render OpenGL properly. If you don't use the QT Painter, then you will likely end up with invisible menus and other graphics related problems.