What's the problem here, with the A/C and the idling engine?

2000 Ford Contour SE.
1. A/c blows the air out from the defrost vents by the windshield instead of the vents on the panel…

2. When I'm stopped at a red light with car idling, the car starts vibrating as if it wants to shut off. I was told the problem was a sensor that's connected to the engine that keeps the engine revved at a steady pace while it's idle. Is that true?

You were told wrong. I'd look for that vacuum leak that is keeping you from changing the vent and it is causing the ruff idle.