I don't necessarily think Vieira deserved another deal, given he put in one good performance (against Aston Villa) but ten or so poor ones. But footballing merit was always one of a number of reasons for his being signed. He's important in other ways, for experience, for leadership, as Mancini's enforcer in the dressing room as well as his eyes and ears.
He won't play too much next year, but he will be important for big European games, particularly if we go 4-5-1. And if Mancini stays he could well grow into the coaching staff.