I would agree that if the motor runs at all in the pump, then the problem lies elsewhere in the system. I had the motor stop owrking on my 190E 2.3-16 and was about to order a new one when the parts supply salesman suggested I run a wire from the battery to the unit and see if it worked. The pump worked. They very rarely fail, and the rest of the vacuum operated clap trap by comparison is kind of flimsy. Mine turned out to be a bad connection (electrical) under the rug on the driver's side just below the clutch. Took some doing to find it but the fix was free.
Finding the problem in the vacuum system can be a pain in the rear, but you can dope it out. Good Luck. Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)