Engine performance is not related to vacuum issues. If you even had no vaccum system at all car would still run fine other than you would be unable to shut it off and have very poor brakes.

Again with someone pressing the pedal down the lever on the injection pump must reach the stop. This is just so common it has to always be checked.

I actually purchased a 240d that the owner had driven this way since he acquired it many years prior to myself. I fixed it in about five minutes when I was picking up the car. The change blew him totally away. I estimate he drove the 240d at about half power for at least several years.. To me that is a challenge I would not personally want.

The pump injection pump lever in the system is not really adjustable. You have to get the linkage in shape so that the injection pump lever hits the end stop with the pedal. Really that is job #1 and very common.