Bartolo , still annoyed with Figaro for having made it easy for the Count and Rosina to marry ( this had happened in Beaumarchaisâ play The Barber of Seville ) , promises to help Marcellina . Bartolo , still irked at Figaro for having facilitated the union of the Count and Rosina ( in The Barber of Seville ) , promises , in comical lawyer-speak , to help Marcellina ( aria : La vendetta -- `` Vengeance '' ) .