My air condition vent has condensation on it and drips into room. What causes this and how can I prevent or stop it ?

The humidity in the room is high enough to cause the condensation. Usually if you run the unit long enough and if it is the proper size the condensate problem will go away since the moisture will be removed from the area.

When warm moisture laden air hits cooler air or surfaces the water/condensate falls out of suspension and water drips happen. This usually happens in the A/C coil in the furnace or air handler. You might check to be sure the condensate drain line is flowing properly in your unit.

I had the same problem and my BF had to put a board under the unit to tilt it a little bit and then it drained outside.