Great to have got in touch. I have the same problem as your w126, and my mechanic located the valve and found if faulty. However, on replacing the valve, a new problem arose. Every time the valve engaged (vacuum controlled by a solenoid switch activated by microprocessor) extra air creates a very lean mixture and the engine stalls. My workshop is unable to explain the phenomenon, and I have had to put the faulty valve back.
1. If the vacuum leak is from this source, plugging the vacuum line should improve the warm start and idle status.
2. There should be no adverse impact from this action since the valve does not work anyway.
3. It is greatly puzzling why replacing a faulty valve should create a new problem.
4. There must be a coexisting fault somewhere else to have produced this domino effect.
I'll keep you updated of further developments!