VNC is an interesting beast on OS X. In Lion, OS X finally gained the possibility of having truly virtual sessions, meaning that they aren't displayed on the console. This is accomplished by enabling VNC, and logging in as a user that is not logged in at the console. That new user will have a private session to work in, and the console user will go uninterrupted.
If you're unsure of what I mean by "console", the console is the one and only session that a user directly attached to the computer via keyboard/mouse/monitor can use. That is commonly referred to as the "console" or "physical" session.
Note that on all previous versions of OS X, logging in as an allowed user could only put you on the console session, possibly colliding with a console user if they were actively working on the system at the same time.
On any other operating system, you can set up a private VNC session using almost any resolution you want, within a certain realm of feasibility. This is unfortunately yet to be the case on OS X. VNC still seems to hook into the physical capabilities of the system, perhaps this happens because the video card is used to render the desktop session even when you're using it remotely. Rendering the OS X animations is very intensive, so offloading them to the video card and freeing up general CPU time is an ideal situation for pretty much anybody.
So the answer? I don't believe you can. You can turn it down via the System Preferences like usual, but the highest resolution you'll get is that which is supported by your video card.