The Vex motors do not work off plain DC current going to the motor. There are three wires there, one +5v, one ground/neutral, and the signal. The Vex motor - being a servo motor - will not work without a signal, which in this case is Pulse Width Modulation. Depending on this signal, this is how you control the output of a servo motor.
And yes, you can gear the Vex motors down to a slower speed with either the Vex gears or the Vex Chain and Sprocket kits.
