To get a sense of the multiple sides of Leonard Bernstein’s creative personality — from the whimsical to the sentimental, the flashy to the intimate — you almost have to go to his vocal music. The range of his songs for the theater and the concert hall hints at the capacious vitality of his compositions. The vibrant American mezzo-soprano Isabel Leonard helped Michael Tilson Thomas and the San Francisco Symphony start celebrating the Bernstein centennial with last week’s appearance in “Arias and Barcarolles.” Now she has the stage and an entire evening to herself, as she offers a mixed slate of Bernstein songs accompanied by pianist John Arida.