Freaky vacuum related issues should consider vacuum pump failures on the OM 60X engines, if it's not something else pretty obvious I'd be checking out the Vac pump before it actually fails with it's often pretty severe consequences.

The brake loss of vacuum is troubling because it's a direct connection between the pump and the booster and it would take a massive leak to effect the loss of vacuum if it was not the pump failing to generate enough vacuum.

As an aside I was recently in contact with another forum member with an 87 OM603 who was telling me his vac pump had completely failed to produce any vacuum as measured at the pump's outlets but as of yet has not exhibited any other signs or sounds of any impending mechanical failure of the pump or it's driver. My advice to him was to not start and run the engine before getting a good look at the vacuum pump's problematic working parts that are inside the timing chain cover space.