Vacuum leaks are reasonably easy to identify, and often easy to fix. Common leaks include injector seals, hoses, and the idle control circuit. Often found with a squirt bottle of water with the engine running. If your injector seals have never been replaced, they're good candidates for leaks as they age harden and crack.
If nothing is known about a particular vehicle, there is a certain sequence to evaluating problems; a logic diagram, if you wish. Once vehicles become better understood, the logic diagram has a different starting point: this is what you pay for with expert help. If your tech knows that 70% of MB 190 16V engines with hard starting problems have vacuum leaks, that's where the diagnosis will start. If that is not the cause of your problems, then a tech willl likely go to the next most common problem before undertaking a rigorous, expensive diagnostic series.
This opens a potentially heated discussion about the proper way to diagnose and treat vehicles, but I always preferred to establish the most likely issue first ( air, fuel, mechanical, electrical) and go from there. This is not intended as a superficial "fix the part and get it out" approach. If there is a blown fuse, what caused the failure? corrosion or circuit overload? short to ground? bare wire? bad motor? A good tech soon learns, especially when concentrating on one brand, which problems usually cause which symptoms, but good techs are also willing to look efficiently for other issues when the common solution is inadequate.
So, if it's a vacuum leak, great. If not, we've already given you several possibilities that may be the actual problem. If the vehicle has not been well maintained, you may have a plethora of problems amid your dearth of diagnosis.