Plácido Domingo, Reneé Fleming, Anna Netrebko, Diana Damrau or Sonya Yoncheva. All these stars will be in the biggest theaters in Europe and audiences will be in for great performances. Austria The Vienna State Opera will celebrate Plácido Domingo’s 50th-anniversary with selections from “Un Ballo in Maschera,” “La Traviata,” and “Simon Boccanegra.” On Saturday the company performs “Das Rheingold” and on {…}