On my W126, that hose is not a vacuum hose, rather it is a "vent" hose for the back side of the regulator.

There is a piston like thing which slides back and forth inside the regulator to adjust fuel pressure. And the back side of that needs to be open to the air so the piston can slide freely. Thus the "vent" hole.

The only thing is, a fuel regulator might at some point in its life leak. And if that happens, it would not be very good if it leaked fuel onto a hot engine. Thus the vent is connected to the engine crankcase via that hose.

(Measure the vacuum - none on mine! Because it goes to the crankcase.)

I would get a new fuel pressure regulator if necessary. Sort of like an insurance policy against a potential engine fire. Also that hose may keep dust and dirt from entering the pressure regulator (as it sucks in air).