what is VAM? On all of my segments the VAM doesn't show up except for when I rode Torrey Pines, but that was with the D group of a bike club in San Diego. We were going annoyingly slow. Why don't the others have VAM?

Here is my only VAM I could find

http://app.strava.com/rides/1143623#17202185

The #1 guy is a pro, at a 1600 VAM.... doesn't mean anything to me because I dont know the meaning of VAM, but other than that he is almost twice as good as me.