Everything Silber has said is correct, but let me add that you missed the first obvious troubleshooting step on any vacuum issue: check the output from the vacuum pump by putting a vacuum gauge onto the main vac line where it enters the brake booster. That assures you that you have a baseline of proper vacuum going into the system,
then you can check to see if/what other components are bad. See:
Mercedes Vacuum Troubleshooting
The VCV in theory is always bleeding off some vacuum, when the car is running, but it bleeds off more as throttle opening increases to where it drops to zero to allow the transmission to shift properly.
To the question that was not answered, the vacuum control valve is held on the top of the IP by two or three small flat-head screws. It is a stand alone item and is not connected to the internals of the IP in any way. Do not remove it unless you have to, and it doesn't sound like you have to in this case.