Make the sauce by whisking together the vinegar, soy sauce, sugar, corn starch, water, and hoisin sauce.
Marinate the chicken in 6 tablespoons of the sauce for 30 minutes.
Cook the remaining sauce in a wok over medium heat with garlic, ginger, and chili flakes until thickened, about 2 minutes.
Whisk the egg whites until slightly foamy.
In a separate bowl, mix the corn starch, flour, remaining marinade, and baking soda.
Dip the chicken in the egg whites, then coat in the corn starch mixture, pressing to adhere.
Fry the chicken in batches in 350°F oil until golden brown, about 4 minutes for thighs.
Toss the fried chicken in the cooked sauce until evenly coated.
