For those who do not know and reference this thread in the future, vacuum is what controls the airflow mixture settings for the air conditioner/heater and is totally independent of the fan speed control. Focusing on the problem at hand, the tree is tough and chances are it is not cracked. However there is a problematic, black vacuum line that runs from the driver side all the way across to the passenger side along the firewall and then into the passenger firewall. This is the only vacuum line that usually causes problems right out of nowhere. It becomes brittle and almost "dry-rot" due to engine heat and age. Sometimes it could be fine, but simply became detached simply by bumping into it while working around it. I am willing to bet your leak is right at the fitting coming off of the main tree. If not there, follow your way down the line checking for breakage or cracks. They are usually obvious. Once you find it, you can cut bad parts off of the bad line and replace whatever connectors you need. Reattach a newly re-cut ends back into the tree with new connectors of the correct size. If the crack is in the middle of the line, cut the pab parts off of the line and insert a bridge connector of the correct size to fill the gap. Be careful not to cut away too much of the line, other wise you will be replacing that whole vacuum line. Still, replacing the whole vacuum line is one of the simplest things to do on that car. You can find a pack of assorted vacuum line fittings of different types and sizes made by Dorman (Makers of HELP!) in almost every auto parts store.