Vices Are in Our Power
Keith Seddon (pp. 40-41), in a most useful book, Epictetus' Handbook and the Tablet of Cebes, explains the way in which vices are in our power:
"Are the passions alone in the category of 'those things contrary to nature amongst the things that are in our power'? Probably not. Stoics describe the disposition of the non-virtuous person as kakos, bad or vicious. All of us — excepting only the Stoic sophos — are susceptible to assenting to false judgments about what is good and bad, and what truly benefits us (virtue), and these assents are, or give rise to, the passions which are impulses to act in ways that are non-virtuous and contrary to our own interests (and thus contrary to nature — that is, contrary to our own human nature, obviously, but also contrary to cosmic nature, since it is our failure to accept the way the world goes that makes us prone to assenting to false judgments). Thus someone who is prey to the passions must necessarily be vicious, and someone who is vicious has got into that state by falling victim to the passions. This gives us reason to place the vices, alongside the passions, in the category of what is contrary to nature amongst the things that are in our power. Although it is not really the vices and being vicious that are directly in our power (though vicious behaviour, to be sure, is contrary to nature), but the judgments to which we assent that either are themselves passions, or give rise to passions (and it is the passions, conceived of as impulses, that motivate vicious actions) — it is assuredly the case that if we can master our faculty of assent we will at one and the same time master our vicious behaviour."
Toward the Core Vices of the Types
Keith Seddon (2005). Epictetus' Handbook and the Tablet of Cebes: Guides to Stoic Living. New York: Routledge.