I had my alternator replaced, and ever since the air flow from the heater/air conditioner/defrost comes out on top and bottom, but no flow whatsoever from the front vents, no matter what the settings are! Has anyone delt with such a problem?

This problem more than likely has nothing to do with your alternator being replaced, unless you have electronic climate control.
What kind of vehicle ?

have you tried checking out your vacuum lines?
they could have accidentally gotten disconncected.