Whisk the vinegar, honey, soy sauce, sesame oil, chilli garlic sauce, garlic and salt in a bowl until smooth. Pour half the marinade into a large snap lock plastic bag and retain the other half of the sauce. Place the chicken thighs into the bag containing marinade, squeeze all the air out of the bag and seal. Shake a few times to coat chicken then refrigerate for 1 hour, turning bag once or twice.
Preheat oven to 220 degrees C.
Pour the other half of the marinade into a saucepan over medium heat. Bring to a boil and cook for 3 to 5 minutes, stirring often to thicken sauce. Remove the chicken from the bag then discard used marinade. Place chicken thighs into a baking tray then brush with 1/3 of the thickened marinade from the saucepan.
Bake 30 minutes, basting one more time after 10 minutes. Cook until juices run clear. Let stand for 5 or 10 minutes. Meanwhile, bring remaining marinade back to a boil for 1 or 2 minutes then serve with chicken. Sprinkle with spring onions.