If you have a good GPS unit and you load your "next" waypoint (on inflatable marks, not easy, but if it's a government channel mark, you can get the lat/long off a chart) then VMG is good to that waypoint. GPS VMG doesn't care about wind or current, just your gain on the next waypoint. You can be going 10 knots at a 45 degree tack with current pushing you back...it only sees the relationship of your unit gaining on that waypoint in VMG mode. You can then try each tack and see which one is getting you there faster, but as Jake said, nearly impossible to load all that stuff while actually racing!
If you are doing a long distance race and can load the lat/long ahead of time, it is useful.