VMG in terms of sailing isn't measured to a single mark, that is just how some non-sailing GPS devices define it.

Even the very first popular sailing GPS, the Velocitek, required two points to be marked that defined a wind angle. One point directly to windward of the other. It didn't matter where these points were marked, as long as they reasonably defined the wind direction. If the wind direction changed, you'd have to mark two more points that defined the new wind direction. The old devices required you to sail to two points and press a button to mark it, but now the latest device only requires you to dial a wind direction into it, no point marking required.

VMG is extremely closely tied to polar plots and wind direction.

This is the best diagrammatic definition of VMG I can find at the moment...
Velocitek VMG

You can easily calculate VMG from a polar plot, just draw a horizontal line from the very top peak and bottom peak of the curve across to the vertical axis. Where they cross are the upwind and downwind VMG targets. Go back to the very top and bottom peaks and read off the actual boat speed to find what most ppl call the target speed, the actual speed you try to sail at to maintain optimum VMG.