The definition as listed at reference.com is to simply take the law into ones own hands or without recourse to lawful procedures and doesn't include that it is unlawful or illegal in its own right. It seems pretty apparent that it can be argued this is attempting to punish individuals for unlawful actions "without recourse to lawful procedures."
You left off the last few words of my comment which stated that it met the definition as you had provided it.
http://dictionary.reference.com/browse/vigilanteism
The definition as listed at reference.com is to simply take the law into ones own hands or without recourse to lawful procedures and doesn't include that it is unlawful or illegal in its own right. It seems pretty apparent that it can be argued this is attempting to punish individuals for unlawful actions "without recourse to lawful procedures."