Add ⅓ cup distilled white vinegar, ⅓ cup soy sauce, 2 tablespoons minced garlic, 1 tablespoon olive oil, 2 packed tablespoons light brown sugar, and 1 teaspoon whole black peppercorns to small mixing bowl.
Whisk vigorously until ingredients are incorporated and brown sugar has dissolved completely. Once sugar is dissolved, add 3 dried bay leaves and gently stir to incorporate. Set bowl aside.
Place slices of 1 medium white onion in one even layer across bottom of Instant Pot. Top onions with 2-2 ½ pounds bone-in, skin-on chicken thighs, then pour vinegar mixture evenly over tops of chicken thighs.
Secure lid on Instant Pot with vent in Sealing position. Set Instant Pot to Manual High Pressure for 10 minutes. Instant Pot will pressurize, then 10-minute cook time will begin.
While chicken cooks, line baking sheet with aluminum foil. Set baking sheet aside.
When 10-minute cook time on Instant Pot ends, carefully Quick Release pressure. Use caution - escaping steam will be very hot.
Carefully remove lid and set aside. Transfer chicken thighs to foil-lined baking sheet, placing thighs skin-side up without overlapping or stacking. Set baking sheet aside.
Set Instant Pot to Sauté. Stir remaining sauce and simmer 3 to 5 minutes or until sauce has thickened.
While sauce simmers, adjust oven rack to approximately 6 inches beneath heating element. Preheat broiler on high.
Place baking sheet under preheated broiler. Broil chicken thighs 6 to 8 minutes or until skin is crisp. When satisfied with texture of chicken skin, carefully remove baking sheet from oven.
Transfer chicken thighs to serving plates. Ladle thickened sauce over tops of chicken thighs, then garnish with sliced green onions and serve.