Not vacuum controlled.
The rear vent for fresh air is cable controlled via a two piece cable. If the slider is difficult to move, either the slider is gummed up or the cable is in a bind. Could also be the lever under the dash is binding.
Try doing a search on "center console 126", or something similar. There should be some posts with detailed steps and pictures.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.