Are Virtues a Convenience?

On Page 187, starting at the word "Suppose" and continuing through the word "attention" Hume makes an argument for the virtuous man who is delivered into a "society of ruffians, remote from the protection of laws and government." In this argument, he is addressing Justice as a virtue, however I wonder if not all virtues are being disregarded in his example. Either way, he argues that a virtuous man, put into such a dire situation is not only forgiven for, but expected to, disregard what many argue is the very essence of 'man,' being the conscious decisions to act morally. Instead Hume says that such a man should revert to a more primitive, and animalistic behavior [a property belonging to the body not the mind] of self preservation. I take this to mean that the man is to literally lower himself to the level of the "ruffians" he lives with in order to maintain his existence.

My question however, pertains to the values of these virtues. If they are not worth a man upholding them until the point of death, then how can they, which are immaterial and completely dependent notions, be worth anything at all. I would argue that better is the man who upholds his virtue, if possible until death released him from such an obligation, merely to prove that we are more than the animal in which we reside.