La sonnambula used themes from a Vincenzo Bellini opera that had a libretto by whom?
La sonnambula (Balanchine): La sonnambula "(The Sleepwalker)" is a ballet by the co-founder and ballet master of New York City Ballet, George Balanchine, made to Vittorio Rieti's music using themes from the operas of Vincenzo Bellini including "La Sonnambula", "Norma", "I Puritani" and "I Capuleti e i Montecchi" (1830–35) and with costumes by Karinska. I Capuleti e i Montecchi: I Capuleti e i Montecchi ("The Capulets and the Montagues") is an Italian opera ("Tragedia lirica") in two acts by Vincenzo Bellini. I Capuleti e i Montecchi:  The libretto by Felice Romani was a reworking of the story of "Romeo and Juliet" for an opera by Nicola Vaccai called "Giulietta e Romeo" and based on the play of the same name by Luigi Scevola written in 1818, thus an Italian source rather than taken directly from William Shakespeare.
Felice Romani