Author Lizzie Vieh’s brilliant play “The Loneliest Number” initially appears to be a slight off-beat comedy about a swinging couple’s encounters but after its first third evolves into a profound, suspenseful and searing exploration of relationships.  Ms. Vieh’s dialogue is sharp, filled with well-crafted jokes and painful depth. A wistful description of a children’s Halloween parade with them in their costumes becomes an insightful reverie of desires.