Objection 1. It would seem that vice is not contrary to virtue. For one thing has one contrary, as proved in Metaph. x, text. 17. Now sin and malice are contrary to virtue. Therefore vice is not contrary to it: since vice applies also to undue disposition of bodily members or of any things whatever.
Objection 2. Further, virtue denotes a certain perfection of power. But vice does not denote anything relative to power. Therefore vice is not contrary to virtue.
Objection 3. Further, Cicero (De Quaest. Tusc. iv) says that "virtue is the soul's health." Now sickness or disease, rather than vice, is opposed to health. Therefore vice is not contrary to virtue.
On the contrary, Augustine says (De Perfect. Justit. ii) that "vice is a quality in respect of which the soul is evil." But "virtue is a quality which makes its subject good," as was shown above (55, A3,4). Therefore vice is contrary to virtue.
I answer that, Two things may be considered in virtue--the essence of virtue, and that to which virtue is ordained. In the essence of virtue we may consider something directly, and we may consider something consequently. Virtue implies "directly" a disposition whereby the subject is well disposed according to the mode of its nature: wherefore the Philosopher says (Phys. vii, text. 17) that "virtue is a disposition of a perfect thing to that which is best; and by perfect I mean that which is disposed according to its nature." That which virtue implies "consequently" is that it is a kind of goodness: because the goodness of a thing consists in its being well disposed according to the mode of its nature. That to which virtue is directed is a good act, as was shown above (Question 56, Article 3).
Accordingly three things are found to be contrary to virtue. One of these is "sin," which is opposed to virtue in respect of that to which virtue is ordained: since, properly speaking, sin denotes an inordinate act; even as an act of virtue is an ordinate and due act: in respect of that which virtue implies consequently, viz. that it is a kind of goodness, the contrary of virtue is "malice": while in respect of that which belongs to the essence of virtue directly, its contrary is "vice": because the vice of a thing seems to consist in its not being disposed in a way befitting its nature: hence Augustine says (De Lib. Arb. iii): "Whatever is lacking for a thing's natural perfection may be called a vice."
Reply to Objection 1. These three things are contrary to virtue, but not in the same respect: for sin is opposed to virtue, according as the latter is productive of a good work; malice, according as virtue is a kind of goodness; while vice is opposed to virtue properly as such.
Reply to Objection 2. Virtue implies not only perfection of power, the principle of action; but also the due disposition of its subject. The reason for this is because a thing operates according as it is in act: so that a thing needs to be well disposed if it has to produce a good work. It is in this respect that vice is contrary to virtue.
Reply to Objection 3. As Cicero says (De Quaest. Tusc. iv), "disease and sickness are vicious qualities," for in speaking of the body "he calls it" disease "when the whole body is infected," for instance, with fever or the like; he calls it sickness "when the disease is attended with weakness"; and vice "when the parts of the body are not well compacted together." And although at times there may be disease in the body without sickness, for instance, when a man has a hidden complaint without being hindered outwardly from his wonted occupations; "yet, in the soul," as he says, "these two things are indistinguishable, except in thought." For whenever a man is ill-disposed inwardly, through some inordinate affection, he is rendered thereby unfit for fulfilling his duties: since "a tree is known by its fruit," i.e. man by his works, according to Matthew 12:33. But "vice of the soul," as Cicero says (De Quaest. Tusc. iv), "is a habit or affection of the soul discordant and inconsistent with itself through life": and this is to be found even without disease and sickness, e.g. when a man sins from weakness or passion. Consequently vice is of wider extent than sickness or disease; even as virtue extends to more things than health; for health itself is reckoned a kind of virtue (Phys. vii, text. 17). Consequently vice is reckoned as contrary to virtue, more fittingly than sickness or disease.
Source: Summa Theologica (New Advent)