Being fully virtuous is compatible with doing some things that are wrong. A juror, call her Alice, while acting in the best conscience, send an innocent person to prison, say because enough witnesses for the prosecution are particularly effective liars. It can even be the case that sending the innocent person to prison is done out of virtue and further contributes to virtue.
But now suppose Bob is a juror brainwashed through no fault of his own into thinking that members of some ethnic group should be found guilty whether or not they really are guilty, and that Bob voted to convict. He may well be acting in good conscience. But Bob is a vicious person, even if, I shall suppose, not culpably so. (If this case is possible, that creates trouble for claim (4) here.) Moreover, even though the only way for him to avoid incurring guilt is to unjustly vote to convict (for then he is inculpable, but if he voted to acquit, he would have been culpable for violating conscience), by voting to convict she contributes to her vice.
It is natural to distinguish the cases of Alice and Bob by noting that Alice's wrongful action comes from ignorance of non-moral matters while Bob's comes from ignorance of moral matters.
But that's not quite the right distinction. For instance, the question whether the accused person was in fact innocent may itself hinge on some moral question. Imagine that a perfectly truthful witness testified that either vegetarianism is not morally required or the accused was holding a knife, and this witness refused to say which disjunct is true. If Alice mistakenly (let's suppose) thinks that vegetarian is morally required, she could thereby come to an error about whether the accused committed the crime, on the basis of a mistake about a moral question.
Furthermore, there is a third case. Consider Carla, a good doctor who needs to make a decision whether to perform some procedure. The moral issues are quite unclear, and Carla cannot figure them out on her own. But Carla has good reason to trust her hospital's ethics committee. She acts in accordance with the committee's recommendation, even though in this case the committee is mistaken. Carla acts wrongly. Moreover, her wrong action comes from a mistake about moral matters, indeed a mistake about the specific moral matter at hand. But Carla's action need not (though this may depend on details of the case) be incompatible with full virtue--morality can be complex, and even a fully virtuous person may be unable to figure out all the intricacies of cooperation in evil or triple effect, say. Given enough complexity in the case, Carla may be sufficiently isolated from the wrongmaking features of her action that she does not become morally worse by acting wrongly.
So if we want to draw the distinction between a type of ignorance of what is to be done that detracts from virtue and a kind that does not, that distinction is not to be drawn by distinguishing between ignorance about moral matters and ignorance about non-moral matters, even if to get out of the vegetarianism case we refine this to be a distinction between ignorance about the morality of the matter at hand and other kinds of ignorance.
So how do we draw the distinction? I don't know.