Jay Leonhart and Josh Nelson 　　UPSTAIRS AT VITELLO’S 　　New York-based Jay Leonhart has been called “the world’s wittiest bassist.” While navigating a career that has included performing with artists as far-ranging as Duke Ellington, Frank Sinatra and Sting, Leonhart also has been composing music and poetry for several decades. Beginning with 1983’s Salamander Pie, Jay has recorded 15 albums with songs dedicated to everything from Wonder Bras to Leonard Bernstein to how the poet Robert Frost made a living (could he go down to the country store and sell a poem, saying, “Here’s a nice one I wrote about the snow”?). Leonhart’s usual one-man show takes on a partner tonight in the form of pianist Josh Nelson, whose deft touch and musical attentiveness should make for a unique evening. —Tom Meek