Vacuum leaks can be located a number of ways, using water mist or carb cleaner around the vacuum connectors and lines will affect idle when sprayed near the leak. The newer the car the greater number of vacuum lines/connections mean longer to locate. Easiest way to tell if it is a vacuum leak is with a vacuum gauge, should see somewhere around 16-17 inches HG at idle (with ACC off), anything less and you have a leak.
I'd say have your mechanic check vacuum at idle, then the O2 sensor, EGR and idle speed regulator.
If you aren't having hard starts after sitting overnight I wouldn't suspect the fuel accumulator.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.