Place diced chicken in a medium bowl. In small bowl, combine soy sauce, vinegar, sesame oil, sugar and red pepper flakes. Pour over chicken. Toss to coat. Cover and refrigerate at least 20 minutes or until ready to cook.
In a large, nonstick pan, heat 2 teaspoons vegetable oil over high heat until quite hot. Add chicken with marinating sauce and cook, stirring constantly, about 3 minutes until chicken begins to brown and is cooked through. Remove chicken with slotted spoon to plate. Try to leave most of sauce in pan.
To the same pan, heat 1½ teaspoons oil. Add onions and garlic. Stir-fry for 30 seconds. Quickly stir in green beans. Add ⅓ cup water, cover, and cook 6-8 minutes or until green beans are tender.
Return chicken to pan. Add 2 tablespoons water and black pepper. Stir-fry until well mixed and heated through, about 2 minutes.
Serve immediately alone or with rice or noodles, if desired.