When the windshield in your car fogs up, it's because the warm, moist air inside the car meets the cold glass and condenses. The heat from the vents can help to keep the windshield from fogging up in the first place, but it won't do anything to defog it once it's already happened.
The only way to defog your windshield is to use the defroster. The defroster works by blowing warm, dry air onto the windshield, which evaporates the water droplets and clears the glass.
