Outstanding vocal performances, many of them by mezzo-sopranos, have been among the defining features of this summer’s musical life. Anne Sofie von Otter finally reached her true potential as Dido in Berlioz’ Les Troyens. The great Anna Caterina Antonacci thrilled us as Cassandra in the same opera, and a newcomer, Kate Lindsey, excelled in the small part of Ascane, going on to greater things (with the splendid baritione, Thomas Meglioranza) in John Harbison’s Symphony No. 5 and, magnificently, in Elliott Carter’s In the Distances of Sleep. Sopranos Lucy Shelton, Iwona Hossa, and, of course, Renée Fleming were equally unforgettable. But one of the most remarkable and fascinating of these took place last Saturday evening at Tannery Pond, when Vivica Genaux, accompanied by Craig Rutenberg, performed an unusual program of works little-known in the classical mainstream.