Actually VMG is your speed made good toward your destination. There is some marine terminology that defines this slightly differently but it all means the same thing. If you are traveling to a destination that is at a 360 degree heading and you are moving at 10mph at a 60 or 300 degree heading, your VMG is 5 mph (and your wasted velocity away from your destination is 5mph). Current and leeway can certainly affect this by changing the course you are sailing over ground (i.e. your heading). The GPS unit knows nothing of tide or leeway or your steering angle (unless you have some REALLY sophisticated electronics) and only knows the course over land you are traveling relative to your destination.

VMG = COS(heading difference to destination) * velocity