vacuum leak

You can get a can of white lithium spray grease and spray in areas you suspect while engine is running at idle, the engine will noticeably slow when the leak sucks the grease in. It is a lubricant so it shouldn't hurt anything. The stuff is also pretty thick and hard to get to light, that is why it works and makes it reasonably safe too. While you at it, lube any hinges or linkages. It is grease and won't hurt a thing. Also, if engine is cool it will wipe right off, after it has been really hot a few times it is a bit hard to get off. As for what a vacuum leak could take to fix, depends on the leak. If a vacuum hose fell off, it's an easy free fix. If it is somewhere else more serious, could be pricey and to very difficult Symptoms range quite a bit by what is effected, cars with vacuum driven ERG valves with a bad hose will often idle like crap. A vacuum sensor could cause all kinds of issues, rich or lean running, timing changes. Note this will only work for leaks near the engine, won't work for checking hoses that may run heater controls, just too far away from cylinders to effect them.