Full vacuum - I would think it is either for the power brake booster or pvc valve if the pvc is not feeding back into air cleaner. Could also have a tee so multiple lines could be attached - power brakes, vacuum canister etc.
If you have PB it does plug into the tree that goes there. I think my AC also connected there. The PVC goes into the port on the carburetor. Here is a pic of one.
If that is what you need and don't have one, pm me.
Spoke too soon. The one in David's pic has the big port on the end and two smaller ports on the sides. Mine has the big one on the end and only one side port.
The big one goes to the power brake booster. The carburetor doesn't need vacuum, the large port on it is for the positive crankcase ventilation (PCV). The small one would probably be for the flapper in the air cleaner snorkel.
You can get the vacuum hose diagram for your car here: