To vary their diets, Vietnamese cooks have devised some ingenious ways to transform rice into other versatile shapes. Consider the rice noodle, the slippery, infinitely adaptable food, is a mainstay of daily dining. Simmered in soup, stir-fried with coconut milk and vegetables, tossed with lemongrass beef or grilled pork, or tucked inside goi kun, Vietnam’s beloved salad rolls. Markets here abound in rice noodles, fresh and dried, skinny and thick