Whisk together vinegar, soy sauce, garlic, olive oil, brown sugar, and crushed peppercorns in a medium bowl until sugar is dissolved. Stir in bay leaves.
Line the bottom of a multi-functional pressure cooker (such as Instant Pot) with onion slices. Place chicken thighs over onion and pour vinegar mixture over the top. Close and lock the lid.
Select high pressure according to manufacturer's instructions; set the timer for 10 minutes. Allow 10 minutes for pressure to build.
Line a baking sheet with aluminum foil.
Release pressure using the quick-release method according to manufacturer's instructions, about 5 minutes. Unlock and remove the lid. Using tongs, transfer chicken thighs to the prepared baking sheet, skin-side up. Select Sauté function and cook until sauce thickens, 3 to 5 minutes.
Set an oven rack about 6 inches from the heat source and preheat the oven's broiler to high.
Cook chicken under the preheated broiler until skin is crispy, 6 to 8 minutes. Transfer chicken to a serving plate and spoon sauce over the top. Garnish with scallions.