Hi.
It does sound like a vacuum problem.
If you disconnect the pipe where it goes to the pump & draw on it &
engine stops, then you have found the problem. The solution is to
check the rubber joiners & check around engine for chafes in the pipe
from the vacuum pump

Diesels are shut down by having a small vacuum powered bellows pull the
fuel rack (inside the injection pump) to its OFF position.
The bellows a/k/a "shut down device" wear out after 15 +/- years or
there's a vacuum leak elsewhere (door lock etc) that lowers the
available vacuum to the shut down device.

These are not terrible complicated issues and can be a DIY project. A
Mity-Vac (may be misspelled) helps diagnose and find leaks. I'd suspect
the shut down device needs to be replaced. Pull the vacuum hose off the
device (located at the aft end of the injection pump), if oil is present
in the line the device is bad, no question.

Don't let this deter you from buying an otherwise good car. The throttle
linkage has a STOP lever to push that will kill the engine until you get
it fixed.