The vacuum starts at the brake booster supply hose. It takes a detour to the proportional leak at the injection pump. The basic line runs over the back of the motor to the modulator.
If in doubt follow the line up from the vacuum modulator. The control is based upon recreating vacuum readings similar to what a gas motor would give. This is 15-17in at idle to zero at full load.
Bosch Master, ASE Master, L1
33 years MB technician