Greg, the short piece of hose joining 1&2 and 2&3 on 73-80 VM series carbs is the way Mikuni vented the carb bowls. The #1 carb has a hole in the top of the bowl chamber but no external vent except for the small cast fitting under this hose. This fitting is essentially just a passage leading to the hole in the bowl chamber top. The hose connects to the #2 carb which also has a hole in the top of the bowl chamber but the #2 has a brass spigot on the side for venting and the hose joins the #1 vent with the #2 vent in going to the spigot. The #3 and #4 carbs vent the same way.
The vent is needed so that vacuum does NOT build up in the bowl chambers otherwise the floats don't work right. If vents are plugged, gas will either POUR out or WON'T flow at all or will flow sporadically.
The smog stuff started in 1979 on the KZ1000 models (might have been on 650s as well but I am not sure). The smog heads had two reed valves that were opened and unburned gunk was blown back into the engine when the reed valves were opened. The opening was controlled by a cannister that was VACUUM controlled that sat above the reed valves and on the front of the engine. The gunk came from the crankcase breather up to the front of the engine where it split into two tubes going into the valve cover reed valve covers.
The gizmo that opened the reed valves was controlled by engine vacuum that came from the carbs from VACUUM spigots you refer to. On the 79/80 KZ1000 carbs, I THINK there was only one spigot and it must have been on carb #3. It looks just like the vents but it was essentially just a hole down into the carb venturi. If the smog stuff is not in place, this vacuum spigot must be capped otherwise it is just an air leak; making steady idling impossible.
