Mix together the soy sauce, rice vinegar, sesame oil, pepper, and cornstarch in a bowl. Place flank steak in the sauce and let marinate. Meanwhile, heat oil over medium heat. Add steak and cook until brown, about 3 minutes. Add onion, garlic, and peppers and cook for 3 minutes. Add marinade to the pan and cook for 3 minutes more. Serve over brown rice.