I've always thought of virtue as the definition"Conformity to a standard of right" although as you can see there are many definitions of virtue and many ideas of what it is. By the way, for the purposes of that definition I define right to be good, humane, etc.
The relationship between happiness and virtue is a little tricky, I think. In a virtuous society, certainly maintaining and striving for virtue can make someone more involved in and in harmony with the people around them.
But what about a virtuous person in an inhumane, deceitful, barbaric society? I'm not entirely certain that being virtuous would lead to happiness. More that it might lead to struggle, possibly personal danger, isolation and generally being at odds with society.
This brings us to the self-sufficient good. Happiness is a self-sufficient good because it is one of the only things pursued for its own sake. If virtue is pursed solely to make someone happy, if being virtuous brings them peace and joy, then even in the barbaric society, it can still bring happiness.