If your trying to get the glass itself out of the frame that holds it, the above answers are a good as it gets.
If your trying to take the vent window out of the door frame, you need to remove the roll up window glass, take the screws out of the forward edge of the window frame, remove the window felts, and then pull the vent window towards the back of the door. It will most likely require a bit of effort. The vent window will "pivot" on a part of the window track that goes into the door.
Clear as mud? Take a close look at it. I think it will become more clear.
-D