There are, as you know, some vices opposed to virtues by a palpable contrast, as imprudence is the opposite of prudence. But there are some vices opposed to virtues simply because they are vices which, nevertheless, by a deceitful appearance resemble virtues; as, for example, in the relation, not of imprudence, but of craftiness to the said virtue of prudence. I speak here of that craftiness which is wont to be understood and spoken of in connection with the evilly disposed, not in the sense in which the word is usually employed in our Scriptures, where it is often used in a good sense, as, "Be crafty as serpents," and again, to give craftiness to the simple." It is true that among heathen writers one of the most accomplished of Latin authors, speaking of Catiline, has said: "Nor was there lacking on his part craftiness to guard against danger," using "craftiness" in a good sense; but the use of the word in this sense is among them very rare, among us very common.
In other words, there are vices opposed to virtues by "palpable contrast" and vices opposed to virtues by "deceitful appearance". Thus, says Augustine a bit later, "two vices are wont to be opposed to one virtue, one that is evidently opposed, and another that bears an apparent likeness." Imprudence is opposed to prudence outright, by palpable contrast, but craftiness is opposed to prudence in the sense that it is a vice that in some sense will substitute for it, so that we think we have prudence when actually we are merely crafty.
Aquinas takes much the same approach to craftiness (ST 2-2.55.3-5). Of course, since Aquinas is Aristotelian, ther eare always at least two vices opposed outright to a virtue, a vice of excess and a vice of defect; and since virtues can be analyzed into their quasi-integral parts, there can be as many oppositions to a virtue as there are parts of it. But he also accepts the Augustinian argument that there is an opposition by false semblance. Like Augustine, he notes that the word can be taken in a good sense, but it can also be taken in a bad sense, as when the Apostle Paul says (II Cor. 4:2), "We renounce the hidden things of dishonesty, not walking in craftiness, nor adulterating the word of God." (Aquinas doesn't explicitly refer to it, but the word astutia is also used to describe the devil's wiles and the craftiness of the serpent in the Garden.) And he also takes astutia, craftiness, as a vice, to be opposed to prudence by false semblance. Prudence is right reason about practical action, and this can be aped in two ways: either the end in view is merely apparently good, or the means used are counterfeit and dissimulating by their very nature. The vice pertaining to the former is carnal prudence, while craftiness is the vice that is associated with the latter. It's important to note that, because craftiness concerns itself with means, it can have a good end in view; but, as Aquinas says, "a good end should be pursued by means that are false and counterfeit but by such as are true."
This vice of astutia is associated with two other names, dolus (usually translated as 'guile' or 'wiles') and fraus (i.e., 'fraud'). Craftiness, like prudence, has two basic aspects: deliberation and execution. These two names simply indicate craftiness in its aspect of execution: guile is craftiness in any form it takes, whether by word or deed (although most commonly by word), and fraud is craftiness in the form it takes when we are crafty by deed.
The vice of cunning, and the kind of education that opposes it, plays an important role in Mary Wollstonecraft's A Vindication of the Rights of Woman. Society has regularly pinned cunning or craftiness (feminine wiles) on women as a vice to which they are especially inclined; Wollstonecraft argues that this is to some extent self-fulfilling and can only be combatted by a real education:
The mind will ever be unstable that has only prejudices to rest on, and the current will run with destructive fury when there are no barriers to break its force. Women are told from their infancy, and taught by the example of their mothers, that a little knowledge of human weakness, justly termed cunning, softness of temper, outward obedience, and a scrupulous attention to a puerile kind of propriety. will obtain for them the protection of man; and they be beautiful, everything else is needless, for, at least, twenty years of their lives.
Even worse, when Wollstonecraft looks at Rousseau's proposals for the education of women, she sees an educational approach that is designed to guarantee nothing but cunning in women. True virtue, however, rests not on cunning but on knowledge, and knowledge requires real learning, and not merely the knack of being able to exploit the weakness of people. Greatness of mind is inconsistent with cunning: greatness of mind requires great views, while cunning is whole caught up in the petty. Men and women must both be educated in ways that aid them in rising above cunning:
To render mankind more virtuous, and happier of course, both sexes must act from the same principle; but how can that be expected when only one is allowed to see the reasonableness of it? To render also the social compact truly equitable, and in order to spread those enlightening principles which alone can meliorate the fate of man, women must be allowed to found their virtue on knowledge, which is scarcely possible unless they be educated by the same pursuits as men. For they are made so inferior by ignorance and low desires, as not to deserve to be ranked with them; or, by the serpentine wrigglings of cunning they mount the tree of knowledge, and only acquire sufficient to lead men astray.
Cunning, in other words, is a degradation of one's rational capacities; and, what is more, it is a degradation of one's rational capacities that leads one to degrade social relations and, at times, the character of other people. A key goal of education must be to give people the larger views, the greater perspectives, that make it more likely that they will rise above cunning, and to give them assurance of their own capacity for free and independent thought. Without this, the need for self-preservation will almost certainly lead to habits by which one's plans are sneaky, rather than open, and manipulative, rather than just or benevolent.