Vacuum lines run to the emission control system, the air conditioner, fuel injection systems, and the idler. A leaking hose can cause any of these systems to fail.
On newer cars, a leaking hose will trigger a sensor to send an error message to the car's computer. This will cause the check engine light to light up, and alert the user of the problem. On older cars, the problem must simply be diagnosed by symptoms. The most common symptom of a leak in the vacuum lines is rough idling or stalling or misfiring when accelerating.
When a vacuum line leaks, check to see that the hoses are all attached. If a line has simply come unconnected at an end, it can be reconnected. More often, the hose's rubber will crack as it ages, if the line is cracked it should be replaced.
