2000 Ford Taurus VENT

There is a source vacuum line that comes out of the firewall where the a/c equipment is, should go to either the engine or a vacuum resevior, be sure the line is hooked up and not cracked/broken. Defrost is the default for a no vacuum situation

Is this something that I can do myself? or should it be done by a mechanic. I called a mechanic today and he said it sounds like a blend door activator and that I was looking at $200-$300 to fix it.

Yes, check the vacuum first, the blend door works hot/cold not defrost to other vents