Assuming that the transmission in the '80 works like my '81 300SD, you do not need to connect the vac system to the vacuum modulator to drive the car (for testing purposes). Without vacuum, the transmission should upshift fine; in fact the system is designed to bleed off the vacuum when you are upshifting. It only needs vac for downshifting smoothly; it will downshift fine without vacuum but there is an unpleasant clunk.

You can hook up a Mityvac to the line that goes to the vac modulator. Drive around with zero vacuum while you are going through the gears; when you get ready to come to a stop, just pump up the vacuum on the Mityvac and it should downshift fine.

If all this works, your problem is the vac system and not the transmission. If it does not work, then suspect the transmission. (Seems like some folks say a bad B2 is indicated if the tranny works OK in reverse but not in forward but not sure.)