Thanks for the suggestion.
So I have now tested each of the three inlets to the driver's door unit. I blocked the other two inlets off when testing.

Test results:
Lock line inlet with plunger in = no vacuum
Lock line inlet with plunger out = holds vacuum

Vacuum source with plunger in = holds vacuum
Vacuum source with plunger out = holds vacuum

Unlock line with plunger in = holds vacuum
Unlock line with plunger out = no vacuum

From this test it seems to me that the unit is working correctly (but of course my logic may be flawed). If so, then where's the leak?