For what its worth, EGR (exhaust gas recirculation) is a helpful way to reduce NOx emissions from many IC engines. By recirculating exhaust, some of the excess air in the engine is CO2 and H2O (both relatively inert), so less N2 is present. Having less N2 means that less NOx can form during combustion. Unfortunately, as most of us know, it can gunk up the intake with a diesel engine. Some have pointed out the gunk does not actually affect performance all that much, so I am not sure what MB engineers envisioned. Perhaps they thought it would stay perfectly clean because new engines have lower PM and lower oily blowby. Or perhaps they knew it would gunk up but figured it didn't matter much overall.

For the OP, it sounds like your EGR valve is functioning. If you don't want that to occur, the cheapest solution is to put a golf tee (or bb) in the vacuum line (and nothing else). A more elegant solution is a block off plate like Rollguy has made up.

__________________
1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).