The vacuum pump diaphragm is something I do not know much about and my mechanic, who is an experienced mercedes guy, did
not mention it. That could be a possibility but from what I have read elsewhere I think the car would smoke significantly in that case. Maybe not. I was originally inclined to think the turbo was leaking but it apparently is not as my mechanic removed the cover and inspected it.

The car has used oil at about this rate during the year and one half I have owned it, although I wasn't actually measuring consumption it early on because I knew it had a oil cooler hose leaking and strongly suspected it needed valve seals because of white smoke on cold startup which went away in a couple of minutes. Once I replaced the cooler hose and had valve seals installed, it no smoked on cold start and has not leaked a drop in my driveway. I even cleaned the engine and checked for less obvious leaks but found none. I bought the car from the second owner who had owned the car for about 4 years and had very few records. I was so certain the car was solid mechanically I did not worry about that. I have never tried to contact the original owner but I may try to do that this week if I can track him down just to see what he says about oil usage.