for I too went through a similar purchase and "BB" in the vacuum line, except mine was in the EGR circuit and when the mechanic removed it as part of his initial going over and valve adjustment and when later the car car ran poorly it was I who figured it out. Mine is a 1980 300D CA car that must have had one of the early EGR systems [my 1980 240D has none at all!]. Keep the tranny's vacuum modulator hooked up... I canNOT comment on "Banjos" as I have none!

I would suggest you temporarily plug the EGR system vacuum line and then look down the throat of the intake and see if the valve is open when it is suppose to be closed. Run the car this way too and note if there is any improvement in the acceleration at 55+ mph. If the car's performance improves, then you have to decide whether to repair or use the 'ol "BB" again but in a different place! [ pssst... the EGR line !]
Sam