Should be a rubber connector, just a bigger piece of black rubber to connect the vacuum line to the actuator. It could have fallen into the door when it came undone, unlikely, but possible. Far more likely that only one side of the connector gave up.

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77' 300D, "Cartman" SOLD @ 150K (didn't know what I had)
83' 300SD, "The Superdon" 325k+ @ 28mpg
95 E320 wagon, "Millennium Falcon" 231k+ @ 24 Mpg
95 E300D, "Sherley" 308k @ 33.69 Mpg, currently anticipating a head
99 Suzuki Intruder "Trudy" @ 45 mpg