The Sarasa Ensemble offers two concerts that celebrate the young orphan girls under Vivaldi’s tutelage, who due to their virtuosic talent put the composer’s name and the city of Venice on the proverbial map. “Vivaldi’s Orphan Girls: Virtuosi della Pietà” is set for Friday, May 14, with performances at 5 and 7:30 p.m. at the BMC. Performers include soprano Kristen Watson; violinists Susanna Ogata, Susannah Foster, and Keats Dieffenbach; Jenny Stirling on the viola; cellists Jennifer Morsches and Timothy Merton;…