The line between finding fame and losing one’s mind is disturbingly blurred in the Vineyard Theatre production of Scene Partners, the latest quirky work from mind-bending playwright John J. Caswell Jr. Operating on as many as four different levels of consciousness, this messy, stratified tale is held together, barely, by director Rachel Chavkin, who utilizes the strongest of glues: a sure-handed and deeply felt performance from her lead actor, Dianne Wiest.