Whisk the vinegar, soy sauce, olive oil, chili garlic sauce and garlic, in a small bowl until smooth. Do not add the sugar yet.
Pour half the marinade into a large plastic zip lock bag and set the the other half of the sauce aside.
Place the chicken thighs into the bag containing marinade and seal the bag removing as much air as possible. Shake the bag to make sure the chicken is evenly coated. Set the bag upright in a bowl to keep the marinade around the chicken .
Refrigerate the chicken at least one hour or longer if you have time. Turn the bag occasionally to ensure the chicken stays evenly coated with marinade.
Preheat oven to 425F
Bring the other half of the marinade to a simmer in a small saucepan over medium heat for about 3-5 minutes or until it thickens slightly.Remove the chicken from the bag; discard used marinade.
Place the chicken thighs into a 9x13-inch baking dish, and brush each with about 1/3 of the thickened marinade from the saucepan. Don't brush directly from the pan so you don't contaminate the sauce with the raw chicken.
Bake the chicken about 30 minutes or until an instant-read thermometer reads at least 165F when inserted into the thickest portion of each thigh.
Remove the chicken from the oven. Heat the remaining sauce in a 10 - 12" saute pan. Add the sugar and stir. When the sauce is simmering add the chicken and saute until the the sauce coats the chicken well.
Plate the chicken on a warmed plate or small platter and sprinkle with the sliced scallions.
Serve with steamed rice (white or brown) and sauteed spinach or bok choy.
You can adjust the amount of sugar/honey to taste. We don't prefer overly sugary foods but if you do you can add. more.
There are many varieties including Chinese (what I used here) Thai, Vietnamese and Korean, to name a few. Each has a different but highly concentrated flavor so feel free to use the one you might have, your result will be different than mine (but that might be a good thing depending you your taste).