In a medium saucepan, combine the vinegar, sugar, salt, oil, and ground black pepper.
Bring the mixture to a simmer over medium-low heat. Stir until the sugar is dissolved. Set the saucepan aside and let the marinade cool completely.
Transfer the chicken thighs to a large freezer bag.
Pour the cooled marinade over the top. Seal the bag and shake gently to coat the chicken pieces.
Marinate the chicken for up to eight hours, turning the bag halfway through to ensure all the pieces are evenly submerged in the marinade.
Preheat the oven to 425 degrees. Line a pan with foil and fit it with a baking rack and spray it with cooking spray.
Drain and discard the marinade. Arrange the chicken on the rack.
Bake for 30-45 mintues until the internal temperature of the chicken reaches 160 degrees F and the skin is crisp and golden.
Let the chicken thighs rest for 15 minutes before serving.