For decades, the acclaimed Peruvian novelist Mario Vargas Llosa has split himself into two personalities: There is Vargas Llosa, the author of dazzling political novels such as “Conversation in the Cathedral” and “The Feast of the Goat.” Then there is Vargas Llosa, the author of two titillating sexual fantasies, “In Praise of the Stepmother” and “The Notebooks of Don Rigoberto.” But now that Vargas Llosa is 79 and has won the greatest literary prizes in the world, perhaps he thought, Why bother?