You have been around this forum much longer than I have so I offer comments somewhat timidly...
I have read so much bad stuff about what happens when vacuum pumps break (oil goes throughout vacuum system) that my own approach would be to simply replace it at the first indication of trouble. You have gotten 175k miles out of this one (assuming its original) and thats not bad.
A new pump costs a bit over $200 and you'd have no worries for a long time.
If you are determined to open it up and try to repair it, there is a bunch of info at diesel forum regarding the various internal components and how to deal with them.
Good luck and FYI, I am about to replace my vacuum pump just to avoid the possibility of it breaking.
'85 300CD @ 144k miles