A vacuum leak will cause all sorts of things to be a little off, but a small one isn't going to cause any sort of catastrophic failure, no. Usually you'll find a vacuum leak under the hood though, not under the car around the exhaust system. There are some exceptions probably, like vacuum lines tied (perhaps indirectly) to lines that scavenge fumes from the fuel tank.
When a vacuum line "leaks" it leaks inward: it's drawing outside air from somewhere it shouldn't. All of the air sucked by the vacuum system ultimately ends up getting burned in your combustion chambers. So if you put two and two together, one way to diagnose a vacuum leak, if you have some idea where it might be in the engine bay, is to lightly mist some starting fluid in the area while the engine's running (preferably cold, so there's not a ton of super-hot metal around the bay). If the vacuum system is sucking outside air through a leak, it'll pick up the starting fluid mist and you'll hear the idle speed change as your engine burns it. Have a fire extinguisher on hand and be careful
