Add the fish sauce, vinegar, sugar, and 1 cup water to a small sauce pot. Bring mixture to a boil.
Take the pot off the flame and add minced garlic and chilis.
Chill in the refrigerator until ready to use. The remaining nước chấm can be stored, covered, in the fridge for at least 1 week.
In a bowl, add your boneless skin on chicken thighs, minced garlic, minced shallot, minced lemongrass, Red Boat Fish Sauce, soy sauce, brown sugar, and black pepper. Mix thoroughly and allow to marinate for 30 minutes.
Preheat your air fryer to 375F.
Add your chicken thighs, skin side down, and air fry for 7-9 minutes.
Flip your chicken thighs and air fry at 400F for 7-9 minutes or until skin is caramelized and the internal temperature is 170F.
Serve over a bed of noodles or rice and drizzle nước chấm (Vietnamese dipping sauce) over your noodles or rice.