There is a “cast in place” “Y” in the vacuum hose behind the ashtray area.
Of the two hoses going through the firewall, the black goes to intake manifold and is the only vacuum supply.
The gray hose goes to the hot water valve under hood. The hot water valve is closed when ventilation system is “off” or in “Max A/C” mode.
The “Y” junction is shown in this picture close to the black/pink star – but don’t remember what the hose colors are.
The picture you have listed (post # 2, above) is from Chrysler, but it doesn’t look correct.
Also, look for this black hose under the hood (yellow arrow).
This picture is taken on driver’s side of car, behind the carburetor and looking towards the distributer. You might have to loosen the air cleaner and move aside a bit to get a good look. This black hose travels about two foot before going through the firewall. If that hose/line is cracked or broken, it will need to be fixed before vacuum reaches inside the car.
BudW