Many Cecilia Bartoli fans have been impatiently waiting for their diva to move forward into a more mainstream Romantic opera-and-song repertory, but apparently the singer is content to remain with her Classical/Baroque specialties. Her only New York appearance this season will be next month at Alice Tully Hall, on January 15, with the instrumental ensemble Il Giardino Armonico, and the program is devoted exclusively to Vivaldi. Most of the music can already be heard on Bartoli's new recital disc for Decca (466 569-2), and her singing there clearly indicates that the mezzo-soprano knows exactly what she is doing. Vivaldi's vocal works, his operas in particular, are still largely unknown, just waiting for skillful, committed, and stylistically aware interpreters to bring them back to life. Well, here is one, a virtuoso singer who revels in every formidable challenge, from the most intricate vocal divisions to long-breathed legato phrases and heroic coloratura fireworks. Hearing it all sung live should be an even headier experience.