In a large bowl, combine the vinegar, soy sauce, garlic, peppercorns, bay leaves and brown sugar.
Add the chicken thighs and turn to coat. Leave to marinate for 15 minutes.
Heat the vegetable oil in a large pan over a medium heat. Lift the chicken from the marinade, letting the excess drip off (reserve it), and brown the thighs on both sides for around 5 minutes.
Pour in the reserved marinade and bring to the boil. Lower the heat and simmer for 10 minutes.
Flip the chicken and cook, stirring occasionally, until the sauce is glossy and thickened to a rich coating, about another 10 minutes. The chicken should reach 75°C in the thickest part.
Serve over cooked rice, scattered with spring onions.