The first part of this essay can be read here
To appreciate how an extended conversation presumably only about Justice might include inquiry related to Virtue and Vice, we must appreciate that Justice is recognized throughout (at least by Socrates) as a type of virtue or, perhaps more specifically, as a component of human virtue. Virtue (from the Greek arete) was considered the complete fulfillment of a thing or potential. If that thing was a Greek man, he might exhibit a number of virtues including excellence in sport and war, able management of his inheritance without resorting to something so crass as labor or mercantilism, and a wide variety of other measurements of a man’s cut (including his desire to behave justly) that all contributed to his overall Virtue or successful fulfillment of his ultimate purpose. So as the conversation turns towards a discussion of individual behavior, the question of Virtue and Vice become central to the inquiry.
Near the conclusion of his argument with Thrasymachus, Socrates draws an analogy between that which is just and good with virtue and, conversely, that which is unjust and evil with vice. This idea becomes magnified in the beginning of Book Two when Glaucon, playing the Devil’s Advocate, takes up the vanquished Thrasymachus’ argument that injustice is, in fact, good and virtuous. Glaucon begins his argument by retelling the story of Gyges, a shepherd who discovers a magic ring that renders him invisible. In the story, Gyges uses the ring to first gain advantage over his fellow shepherds and, ultimately, to murder the King and take his kingdom. Glaucon ties this story in with their current inquiry by taking it one step further.
Suppose now that there were two such magic rings, and the just put on one of them and the unjust the other; no man can be imagined to be of such an iron nature that we would stand fast in justice…Then the action of the just would be as the actions of the unjust…And this we may truly affirm to be a great proof that a man is just, not willingly or because he thinks that justice is any good to him individually, but of necessity, for wherever any one thinks that he can safely be unjust, there he is unjust. 
Glaucon’s argument is carried one step further by his brother, Adeimantus who points out that “the universal voice of mankind is always declaring that justice and virtue are honorable, but grievous, toilsome, and that the pleasures of vice and injustice are easy of attainment and are only censured by law and opinion” .
In order to test the validity of their argument (something Socrates himself doesn’t get to in books one and two of the Republic), we must have some certainty about the meaning of the terms under consideration. Virtue and Vice are too easily mistaken for doing good or doing bad. The trouble with the shallow, contemporary reading is that good and bad are merely polar markers that delineate a wide range of behavior that is rarely all one to the exclusion of the others. A man who, for example, robs a bank in order to feed his family has clearly committed a crime but his actions are neither fully good nor fully evil; if, then, the road to hell is paved with good intentions so also would the banister on the stairway to heaven be gilded with evil.
Fortunately, Virtue, in the Greek mode of thinking, has nothing to do with any of these nebulous and unknowable quantities. When Plato speaks of Justice being a Virtue, it is understood that he is saying Justice is but one component of the state of Human Virtue. Justice alone will not make someone virtuous but it may be said that no true Virtue might be complete without it. If we are to accept the proposition that humanity is somehow unique among the animal kingdom from which we arose (hardly a settled matter but I think Socrates and Plato would agree), then we must also accept that the Virtue of a Human Being is unique as well and might well be recognized, as someone once said, by the fruit that it bears. When humanity does as Thrasymachus and the sons of Ariston suggest, practice injustice in whatever degree it can be perpetuated without being punished, it is no different from the law of the jungle where the strongest and smartest flourish at the expense of the weaker and not-so-smartest. If one can suppose that all culture, all faith, all love, all art, all whatever makes us human, can be boiled down to mere elaborations on our biological scripts (the same scripts that every other living thing on Earth is exercising right alongside us), then perhaps this is the more persuasive argument. In fact, regardless of what other baggage we may or may not have picked up along our evolutionary journey, those impulses certainly still exist within us and, at times, may still dominate our behavior.
If, however, we are to consider ourselves as something other than just unusually adaptive primates that exploited the right niche at the right time and accidentally wound up dominating the entire planet, we must also accept the possibility that our Virtue, the full expression of our purpose, might reflect the awesome responsibility of this higher consciousness. These components of Virtue (Justice, Mercy, Love, Reason, pick three of your own, etc), in contrast to the Virtue of any given animal, including the one we used to be, would seem alien and counterproductive. Their relative youth in comparison to the many calls of the wild that reside within in would also render them relatively fragile. Plato quotes Hesiod in observing that “Vice may be had in abundance without trouble; the way is smooth and her dwelling place is near. But before Virtue the gods have set toil.”
The message is clear. Virtue is an attainment of distinctly human potential while Vice, in whatever form it might take, embraces the old laws of survival that come so easily to us but which we execute with a savagery and wanton destructiveness without peer among forms of life we consider to be less evolved.