For dressing, in a small bowl combine rice vinegar, salad oil, brown sugar, chopped lemongrass, and sesame oil; set dressing aside. Divide lettuce among 4 individual serving plates. Arrange shrimp, avocado, and mango atop lettuce. Drizzle each serving with 2 tablespoons dressing. (Chill and reserve remaining dressing for another use.) Sprinkle with green onion and cheese. Makes 4 servings.