Your vacuum seems about right. If anything you might have too large a drop when the proportioning valve cuts in (the 20 to 9 in drop). This would cause the opposite shifting problem though so I would say your problem is in the electric kickdown.
This is done by the small switch under the gas pedal and I don't remember where it goes from there on a 240D. On gas cars it is handled by the fuel pump relay, but I forget on yours. It will be apparant on the wiring diagram.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician