This is one of the symptoms I have in my '80 300 TD which I believe has developed a dead vacuum pump. The transmission does shift up, but only after the engine revs up a lot higher than usual, depending on accelerator pressure and driving conditions of course. In my case, other symptoms are: engine won't shut off with key; hard braking; door locks don't work; engine oil showed up in air filter canister (dry paper type). All these things happened at the same time.
Another reason I'm interested in vacuum influence on transmission operation is the "twin" '80 300 TD that was given to me last month because "the transmission slips". I'm wondering if it has a real serious transmission problem or not. The twin has some problems that are definitely vacuum-related, so this is something I'm going to be getting into soon.