In 2007 Simpson became a member of Steve Vai's band, String Theories. The band began a world tour in June 2007. Simpson received a gold sales award for her participation in Vai's "Where the Wild Things Are" DVD. Her performance with Vai on "Now We Run," was nominated for a Grammy Award in the category of "Best Instrumental Rock Group Performance." Vai commented about her performance at that event: "She should have been the one who had a full performance on the show, because she would have stunned people."