Vigilantes are often described as "taking the law into their own hands." Wikipedia defines a vigilante as someone who, "illegally punishes an alleged lawbreaker, or participates in a group which metes out extralegal punishment to an alleged lawbreaker." The law, being a state institution, is not the motivation for direct action. Anarchists realize that the law is often unjust, and that theoretically-sound regulations are often not enforced, anyway.
Direct action is usually taken against specific pieces of infrastructure or equipment to prevent its use for exploitation or destruction, rather than a person. Thus the reasoning behind it may have less to do with justice than for, say, saving the life of would-be test animals or sabotaging a factory whose working conditions are unbearable.
This distinction is not sufficiently satisfying for me, but maybe someone can use it as a starting point.
