Heat vegetable oil in a large skillet over medium heat. Stir in chicken, and cook until the pieces are no longer pink in the center, about 5 minutes. Remove chicken, then stir in the red curry paste and sugar; cook and stir for 1 minute. Add the bell peppers and onion to the pan, and cook for 2 minutes. Pour in the coconut milk, return the chicken to the pan, and bring to a simmer over medium-high heat. Cook until the onions and peppers are tender, about 5 minutes. Stir in the fish sauce and serve.