The vacuam pump is very long lived. I just replaced the original on my 240D a few weeks ago. It had over a half million miles.
The other vacuam components are the same on the gas cars.
You probably just have one of the locking diaphragms that needs replacing, they are readily available and inexpensive.
Have you checked all the vacuam line connections and rubber fittings near the oil filter area? It is not totally uncommon for a line to knocked loose here when the oil filter is changed.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in