The resolutions visible on this list are the ones that can be handled by the graphical card
and the screen on the computer you connect to.
VNC is not a display manager, replacing the graphical card. It will only copy what you have on screen. As such, you can't change the resolution for something matching to the computer you connect with, if this resolution can't be handled by the target computer. It has to be a valid resolution for this computer, or it won't be displayed.
Edit: Ok, in your case, it doesn't depend on screen, indeed, since there is none. I'm not fully sure then how is it handled when there is no screen, if all resolutions from the graphical card are available, or rather almost none.
Edit2: Give a try to PowerStrip. It's not guaranteed that your graphical card will accept it, but if it does, it will allow you to choose any resolution.