Why does AC only blow out of the floor and not the vents

I saw an answer to a similar question, where the AC was only coming out the defroster. The tech said "most likely it was a loss of vacuum to the HVAC control head. There is a vacuum line that runs through the firewall near the heater case. Make sure that it isn't damaged or disconnected. You may need to pull the doghouse for easier access to the tubing at the intake manifold." I have the same problem, except the air only comes out the defroster. I'm going to try checking the vacuum line as suggested. Hope this helps you as well as me!