The leak could be anywhere, but the two most common, and easiest places to check are; the four way connector next to the oil filter. It is in the way when changing the oil filter and commonly gets jarred loose, and the tee from the big line that goes to the brake booster, this is the main vac source.
If you had a serious leak, you're engine wouldn't shut off, so you need to run down the accessories you've already to discovered to be nonoperational. If you don't find something simple, get a vacuum schematic and a vacuum test pump from the Auto Parts store.
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in