You ask the age old question concerning proactive replacement of a part prior to failure. let's face it, vacuum pumps are mechanical and not even a seer or prophet can predict MTBF (mean time between failure).

Since the consequences of a vacuum pump failure could be fatal to the engine, my recommendation is replace it so you can get some sleep. I'm planning on doing the same replacement very shortly. A long block 617 diesel is around $7,000 while even a used 617 engine isn't cheap.

Pay a little now or possibly pay a lot later.