it sounds like your describing the vacuum canister for the A/C climate control system, and the Emissions Charcoal canister for the fuel vapor recovery system.
the transmission should not be hooked up to Either of those.
the c6 transmission modulator should be hooked up to full manifold vacuum on the intake manifold just like the vaccum booster for the brakes.
the vacuum canister is a vacuum battery of sorts it stores vacuum for use by the climate control vacuum motors behind the dash board.
the canister compensates for the loss of vacuum when accelerating, there is a one way reed valve on the vacuum can the closes when vacuum is lost.
the transmission modulator needs the signal from full manifold vacuum rising and dropping so it knows the load is changing from the engine and it can shift correctly so you want the modulator to see vacuum rise and fall , by having it hooked to the vacuum canister it is seeing constant vacuum which the transmission thinks the engine is under very low load and it will not shift correctly until the mechanical kicks in.
as for the evaporator can for the fuel system.
there is one line on the fire wall that feeds into it from the fuel tank. a small hose went to the original carburetor fuel bowls to allow the vapor fuel in the bowls to not escape into the air. the Vaccum that drives the charcoal can came from a Large paper tube hose that ran into the original air cleaner base, this provided a atmospheric vacuum that sucked air from the canister which vented the vapor build up from the fuel bowls and the fuel tank, back through the intake which the engine burned
hope this helps
this is my car
the black can on the shock tower is the vacuum canister
the blue can on the lower apron is the fuel vapor canister.
the black vacuum can, has 2 hoses, a input from full manifold vacuum on the engine, which goes in the center and has the reed valve, and the output hose which goes through the firewall into the cabin and feeds the main climate control vacuum hose manifold that we call the octopus.
a view of the octopus behind the dash