Having again read Amy Peikoff's essay 'A Moral Dynamiting' in Essays on Ayn Rand's "The Fountainhead" I wonder if Roark had the right to blow up a building. Given the fact that he had no legal recourse available and no way in which to be compensated for breach of contract or copyright infringement, is it still ok for Roark to take matters into his own hands and blow up a building? Is this a form of vigilante justice? Roark has every right to be upset with the outcome of events, that the product of his mind and work has been stolen from him and altered without his consent, but doesn't that mean in this case he has to live with the consequences, doesn't he have to work/live within the law even if the law is wrong? Legally doesn't he have to just walk away because there's nothing he can do? Are Roark's actions an example of an individual taking the law into their own hands and in this case putting others at risk?
Yes, by definition it is vigilante justice--Roark is acting outisde the legal system to vindicate his rights which have been violated. The more interesting (and difficult) part of your question is whether it was right for Roark to do what he did.
As a general proposition, vigilanteism is wrong. Retaliatory force must be subjected to objective rules of law, no matter how clear it is to the victim that their rights have been violated--this is the very essence of why we need a government. Ayn Rand said:
“The Nature of Government,” The Virtue of Selfishness, 108 (available in The Ayn Rand Lexicon) (emphasis added)
However, all principles depend upon a context. The context that makes vigilanteism wrong includes the existance of a government capable and willing to objectively enforce rights. When there is no such government, then one is left to defend oneself the best one can. In such a case, it would not be wrong to take justice into your own hands, i.e. be a vigilante.
Of course most governments are a mixed case--they are partly objective, partly not; partly rights protecting, partly rights violating. Rather than a binary good or bad, governments lie somewhere on a continuum. At some point on that continuum things become so bad that vigilanteism ceases to be immoral, but it is excedingly hard to determine where that point is.
I tend to believe that things were not so bad in Roark's world as to make it okay to do what he did. Rand could have laid a foundation for how bad things were so that we could draw the opposite conclusion, but I think she didn't because the theme of the book was not political. As such, focusing too much attention to the political topic of vigilanteism would detract from the logical progression of the theme of the book. Rand's later thoughts, when focused on politics (as indicated in the quote above) make clear that she is in principle against vigilante justice.