A stranger is drowning in a crocodile-infested river. To pull him out, you'd need to go in the water yourself, and there is a moderate chance (say, 25%) that you would be eaten. You have no dependents to whom you owe it not to risk your life, but of course you don't like being eaten by a crocodile. It would be praiseworthy for you to engage in this heroic act. But if you don't do it, you aren't doing anything morally wrong. I want the story to be set up so this is clearly a case of supererogatoriness.
You have decided not to do it. But then the stranger offers you a million dollars. And so you leap in and pull him to safety.
You're not particularly morally praiseworthy. But have you done anything morally wrong in acting on the mercenary motive? Nothing wrong would have been done had you refused to take the risk at all. Why would it be wrong to do it for money? Indeed, is your case any different from that of someone who becomes a firefighter for monetary reasons? But wouldn't it be odd if it were permissible to be a businessman for profit but wrong to be a firefighter for profit?
So the mere presence of a mercenary motive, even when this motive ia difference-maker, does not make an action wrong. Nor does this constitute the agent as vicious.
But what if the mercenary motive were the only operative motive? That would be constitutive of vice. There need be no vice if the decision whether to save another at significant risk to self is decided in favor of caution, and there need be no vice if money overrides the caution. But if the mercenary motive were the only motive, then that suggests that had there been neither danger to you nor promise of payment, you wouldn't have pulled out the stranger, because you simply don't care about the stranger's life. And that's vicious.
It is morally important, thus, that care for the stranger's life be among your motives, even if the mercenary motive is necessary (or even if it is necessary and sufficient) for the decision to save. Likewise, if you decide not to save, the motive of care for the stranger's life had better be operative. The decision had better be a conflicted one. Only for a vicious person would the decision not to save be a shoo-in.