Anne Washburn’s The Burning Cauldron of Fiery Fire, making its world premiere at the Vineyard Theatre, defies categorization. The offbeat exploration of an agrarian California commune bordering on a cult might also defy comprehension; your mileage may vary depending on your tolerance for unknowable mysteries. There is something mesmerizing about Burning Cauldron, beautifully strange and compelling, with silliness and menace existing so comfortably side by side. Who needs easy answers when the questions are this much fun?