Where is the line between justice and vigilantism?

When is it moral to take the law into your own hands -- meaning pursuing, detaining, and/or punishing criminals as a private citizen? Suppose that you know -- without a shadow of a doubt -- that some person committed a serious crime against you or a loved one. If the justice system cannot punish the person due to some technicality, is it wrong for you to do so? If you're caught, should a judge or jury punish you, as if you'd committed a crime against an innocent person?