W123 manual shut off lever not working but vacuum is??
 

The manual shut off lever is not working, however the vacuum shut off is working when you turn the ignition key to off.

I tried to manually press down on it but no luck, I even disconnected the linkage from the throttle. It sputters a bit and sounds like it will die.

Is the lever slipping on the rod? There might be a screw you can tighten.

If you have an MW Fuel Injection Pump inside of the idle Adjustment Screw is a spring loaded Plunger. When you apply the Manual shutoff the Throttle lever is supposed to depress the Plunger up into the Idle Adjustment Screw.

If there is rust inside of the Idle adjustment screw the Plunger travel is limited and you throttle lever cannot move far enough to shutoff.

If you back off the Idle Adjustmen Screw can the Manual Shutoff shut the Engine down?

What is your idle speed?
Is your Idle speed set by the Idle Speed adjusting screw or is your idle speed hanging on the Governor Idle/Rack Damper Screw? (If you back off your Idle Adjustment Screw and your idle remains the same you are idling on the Rack Damper Screw.)

There is a vertical adjusting screw that uses a locknut which a 9/16 deepsocket fits over. Below that vertical idle adjustment screw, I see a small plunger and it is rusty.

I tried turning down the idle all the way and I still cannot manually turn off the engine.

Tachometer just stopped working but it was at around 950/1000rpm when last working. However it idled normally then after you stepped on the gas it would go to the high idle.