lets say you are sailing to a windward mark and following your VMG on your GPS. If you get to the point wherein VMG becomes zero then negative, aren't you at the layline for that mark and should therefore tack? (not counting leeward drift). It would mean that if you tacked 90 deg then you would be headed straight for the mark. Am I wrong? We do this occasionally when racing on a monohull.
I think the point is that when using GPS calculated VMG one has to remain aware that as the angle of the bow to the mark becomes large (closer to 90 degrees or greater), VMG will drop - as you said, to zero or negative -
Essentially GPS VMG doesn't tell you how fast you are going in the direction that you need to go in order to make a mark. It only tells you how fast you are going towards a mark (or waypoint).
In response to the situation that you described - a negative VMG reading, or a VMG reading of zero does not always mean you are at your layline. Currents, for instance, can have significant effects on how far you have to sail "past" a mark in order to lay a mark. I can recall several instances where tacking/gybing at a GPS VMG reading of 0 would never get you to the mark.
But, yes, discounting all of the common exceptions (current, obstructions, etc.), sailing past a VMG reading of 0 means sailing beyond the layline. But in general, except on the last tack, one would want to tack before VMG dropped to 0 or below in order to stay inside the course.
