How do we know that A, by stepping into coercive shoes, is accurately enforcing the will of the government? This raises the problem of interpretation. What if A is enforcing his own preferred interpretation of the obligations apparently imposed on B, and not the actual obligation imposed on B?To slightly rephrase the question: what degree of legal certainty or expertise must the vigilante obtain in order to enforce the law?
Obviously, there are both easy cases and hard cases. But even then, the person with the inferior position in an apparently easy case will often argue that the situation actually presents a hard case, where the result should be the counterintuitive one--i.e., the person with the apparently inferior position should win. Whether that argument does or should work will depend on your legal philosophy. The next level of abstraction, if you’re interested, would be to express skepticism that A can ever have the expertise or the authority to decide whether he is experiencing a hard or an easy case.
A way to approach this, from a legal standpoint, might be to look into the precise circumstances where "citizen's arrest" is legally justifiable, and, on the obverse, to consider the citizen's duty to report crimes. Both situations presuppose some baseline familiarity with the law's latitude, and both may require the sort of fine-grained judgment that, in the absence of government enforcement, might justify vigilantism.
There's more over at Mr. Wall's blog. Leave your comments there. (I, for one, hope he continues mining this vein.)