That second black line may need to be connected to the intake manifold, so both may go to the reservoir.
You will need to find/access the vacuum reservoir, it will have 2 vacuum lines
One goes to the intake manifold, the vacuum source for the reservoir
The other is the line that goes thru the firewall to climate control
The vacuum reservoir does have an IN and OUT so lines must be connected correctly
IN is for the intake manifold line, there is a check valve on that port like the one on the power brake booster, so reservoir holds vacuum even after engine is shut off or accelerating(very low vacuum in intake)
A Common complaint is that vents in the cab change to Defrost when accelerating, that usually means vacuum reservoir is leaking or check valve is, so when accelerating there is no "reserve" so vacuum level drops in the cab and vents go to default position, which is Defrost windshield vents