I would also ask what kind of heat you get out of the other vents in the system, such as the floor vents and the dash vents?
I only ask because the defrost vents (by the windshield) are just for that - defrosting the windshield. When you set the control settings to one of the defrost modes, that will also turn on the compressor, which also turns on the evaporator - the part that is really the air conditioner. The purpose of this is to remove the moisture from the air, which helps to defrost the windshield quickly.
If the engine is heating up normally (to about 190 degrees F), and you are getting flow through the heater core, then setting the heater controls to about 3/4 on the temperature control, and the air control to "split" (floor & dash), then you should be very warm and toasty in a few minutes.
If you Venture is like mine, you may notice that there is hot air outlets UNDER the front seats, so make sure you don't have anything blocking those outlets. This is where most of the heat comes from for the folks in the back of the van (the kids?).
Please let us know what you have going on there.
Take Care,