Bring forth the evidence! The authorities are not omnipresent and information gathered by citizens of a crime should be brought to the proper authorities' attention for appropriate legal action. Sometimes the authorities need convincing so the hew and cry is raised until said authorities are raised from the inertia and prodded to take said action! No? Such is not vigilantism.

Unfounded and continual harassment of an individual, suspect or not, should be and is illegal. When a mob, organized or not, takes the law into its own hands, THAT is vigilantism.