In a diesel, any vacuam leak or leak in intake system should be fixed because it allows dirt in the engine. Not a good thing.
However, a diesel is not throttled, it takes a full gulp of air at every intake stroke. The engine is controlled by the amount of fuel injected into the cylinder. This shouldn't cause fuel mileage problems. It doesn't sound like you're describing an EGR valve.
When you lost the belt, did the engine overheat?
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in