Yes, vacuum looks can cause hard shifting, particularly downshifting. Plug the lines. Yellow/red line is the doors. Plug it in the engine compartment.
If the center vents have heat, someone has probably wedged them open because without vacuum they should be closed. Still could be a leak in there. Can reach and plug that line through the glove compartment.
I have a leak in my foot well vent diaphragms so when I have the heat on, my TD downshifts hard. So I have to remember to turn the heat to defrost before coming to a stop.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13