Whisk vinegar, soy sauce, garlic, olive oil, brown sugar, and crushed peppercorns in a bowl until sugar is dissolved. Stir bay leaves into the sauce.
Clean and cut up whole chicken is small sizes.
Line the bottom of a multi-functional pressure cooker (such as Instant Pot) with onion slices. Place chicken on top of onions and pour vinegar-soy sauce over top. Close and lock the lid.
Select high pressure according to manufacturer's instructions; set 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 to the prepared baking sheet, skin-side up. Select Saut 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.
Place chicken under the preheated broiler, and broil until skin is crispy, 6 to 8 minutes. Transfer thighs to a serving plate and spoon sauce over the top. Garnish with chopped scallions.
