Don't know about the MityVac, I have a Actron hand vacuum pump and a separate combination vacuum gauge/fuel pressure gauge. With the hand pump I can pull almost 25" HG in a few squeezes of the handle, and with the reservoir out of the loop, I can cycle the locks with a few pumps too. As long as the MityVac can pull about 15", I think it should be just fine.
I just thought of one other thing to check first. The check valve(s). Mine was going bad and would lose vacuum intermittently. I found that I could pull a vacuum on the valve, it would hold, but if I shook the valve the vacuum would bleed off. Changed the valve and then moved on to the rest of the system. On a gas engine the check valve should be between the intake and the first junction. On a diesel I guess it would be between the brake booster and the vacuum pump???.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.