Epistemology is a branch of philosophy focused on knowledge, how we know what we know, and how our internal lives (and what we think, believe, our consciousness, etc) interact with reality.
Virtue epistemology is a sub-field within epistemology oriented around ways of thinking that would seem to result in better—in the sense of being more aligned with reality—understandings.
So while you could establish a sense of truth using hardcore, borderline mathematical rationalist models, virtue epistemology says that someone who has epistemologically virtuous traits—like honesty, humility, objectivity, wisdom, contextual understanding, attentiveness, and creativity, among others—will be just as, if not more likely to arrive at a rational truth (a well-reasoned and accurate sense of reality).
The flip-side of epistemic virtue is epistemic vice.
Viceful thinking, in this context, is thinking shaped by dogmatism, gullibility, intellectual dishonesty, self-deception (blind or recognized), superstition, wishful thinking, an unawareness of one's own biases and prejudices, superficial thinking, and willful naïveté, among others.
In other words, while virtuous thinking, according to this philosophical model, is more likely to help us arrive at an understanding of reality that's close to accurate, viceful thinking will distort our perception of reality, and consequently leave us unable to function as informed, conscious, purposeful beings.
We might believe we're thinking and behaving rationally, and might even apply logical theories that seem to show we're correct and virtuous and the only ones who understand what's going on. But because we've got these viceful traits at the base of our thinking, everything we build atop that foundation is untrustworthy.
This is one facet of one philosophical model for thinking about one aspect of life, and thus probably shouldn't be used as the framework of a fully realized, holistically inclined mental model of reality and self and how we should organize our understanding of reality.
That said, it can be a useful heuristic for understanding what sorts of thinking-related approaches and traits are more likely to serve us, and which are more likely to distort our perception in unhelpful ways.
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