I wouldn’t be ready to condemn the BCDD quite yet, and I am confused by a couple things in your original post...
First thing is that there is only one externally visible connection to the BCDD, and that is an electrical one. There are a couple vacuum connections made to the BCDD, but none of them are externally visible. They are all made internally to the throttle body and are not visible without peering into the throat of the throttle body.
So when you say "the BCDD was disconnected and lines running to and away from it were blocked off", if you're talking about some vacuum lines being capped... Those vacuum lines originate from the throttle body, but despite how close in proximity they are to the BCDD, neither of them have anything to do with it. One of those vacuum lines should be connected to your distributor and carbon canister, and the other vacuum line is used for the EGR system. So if you're having troubles after messing with those vacuum lines, it's not the BCDD's problem.
If you are talking about the electrical connection to the BCDD, that connection is a kind of "dead man switch" such that when that wire supplies power to the BCDD, the BCDD is deactivated and should have no impact. But if that wire was disconnected, your BCDD was active at all times.
Second thing is about you hearing a vacuum leak after you turn the car off... As soon as the engine stops turning, the manifold vacuum goes away, and there are two places where that vacuum is "stored" while the engine isn't running. Those two places are in the HVAC system, and in the power brake booster. Both of those systems should have a check valve in series with them to prevent that vacuum from bleeding off, but I'd start looking there.