Since the July 15 slaying of fashion designer Gianni Versace, Vanity Fair contributor Maureen Orth has been called upon in the media to comment on Andrew Cunanan, Versace's alleged killer and a subject she has been researching for months. The fruits of Orth's research will appear in VF's September issue and also, it turns out, in a forthcomiong Delacorte book. Executive editor Tom Spain acquired world rights from Orth's agent Amanda Urban and plans a spring 1998 publication.