In a large sauce pan, heat oil, medium high. Add chicken, skin side down and cook until skin is golden and crisp. Flip and cook 2 minutes. Move to plate.
Toss all but 1 tbsp fat. Add garlic and cook until fragrant. Stir in vinegar, soy sauce, pepper flakes, and sugar. Return chicken to pan, skin side up, cover, reduce heat to low, cook 15 minutes. Uncover, raise heat to medium and reduce sauce (about 10 minutes).