What is this and how should the vacuum line be routed?
That is the idle up for the AC. The hose for that comes from, or goes to, a solenoid that is mounted on the firewall (yours doesn't have that solenoid on the firewall though). Don't worry about hooking anything to it, since you don't have AC. You can even remove it and remove the stuff from the carb shaft, if you want.
The AC idle up diaphragm is not shown on the vacuum diagram you are looking at.
The hose that you have connected to it now is supposed to go to the rear port on the airbox lid.
Did you check to see if all the vacuum diaphragms are good? The choke pull off is opposite of this on the other side of the carb.....they are usually bad. The HIC is a lot of times bad too (it is towards the front of the carb on the drivers side). The idle up (it is under the choke pull off diaphragm on the drivers side of the carb) is usually good. If any of these are not working correctly, then it can make the engine run crappy.