Hobiegary, seems that your gps calculates VMG by measuring the distance to one waypoint, calculating the velocity from the changes in this distance to the waypoint. This is not VMG as it is used in sailing. In sailing it would be the proyection of the real velocity to some diection (i.e. wind direction), the velocitek s5 seems to do such a calculation.
What?! I don't agree.
The description of VMG that Velocitek provides is no different than what Hobiegary described. Velocitek simply chose to only illustrate the optimal situation that Hobiegary described:
"when you are on a direct (final) approach (sailing right on the lay line)."
Other than that Hobiegary is right - if you want to know your best speed to a target you need to have data that accounts for wind direction as well.
The Velocitek seems to offer nothing more innovative in the realm of VMG calculation than any other GPS that calculates VMG - at least according to the description that they offer themselves:
http://www.velocitekspeed.com/VMG.html 
