In a bowl, whisk the brown sugar, fish sauce, water, rice-wine vinegar, soy sauce, ginger, garlic, black pepper, and red pepper flakes.
Heat the oil in a large frying pan over medium heat. Add the scallions and cook until soft, about 3 minutes. Add the thighs and the brown sugar mixture. Turn the heat to high and bring to a boil. As soon as it comes to a boil, reduce the heat to low and simmer, turning the thighs occasionally, until cooked, 25 to 30 minutes.
Remove the thighs from the pan and cover with foil to keep warm. Increase the heat to high and reduce the sauce by half or until it is slightly thickened and resembles a bubbling caramel sauce. Serve the chicken with the sauce, garnished with the cilantro sprigs.