In order to establish VMG, you first much tell the unit the wind direction by clicking a button at a downwind point, then clicking a button after sailing to a point directly upwind. IMHO, this would be very difficult to do accurately and/or in a timely fashion before a start - especially on a lake where the wind shifts regularly and often dramatically.
True, if you're course is "square" to the wind .... If you had time to mark "C" and "A" buoys (or anywhere along this line that is a sufficient distance apart for the GPS to calibrate) wouldn't the VMG be displayed as the fastest point of sail to this (imaginary) line?
Ex: If the wind is backed then port-tack is favored and your VMG will be very high. When tacking to stbd-tack, you're VMG will be much lower. Compared to a perfectly square course you'd expect your VMG on either tack to be similar.
