In a bowl, whisk together the soy sauce, white vinegar, water, and brown sugar until the sugar dissolves. Set this adobo sauce aside.
Set the Instant Pot to saute mode and heat the vegetable oil. Season the chicken thighs with a pinch of salt and place them skin-side down in the pot. Sear for 3 to 4 minutes until the skin is golden and crispy. Work in batches to avoid crowding. Remove and set aside.
Add the sliced onion to the pot and saute for 2 minutes. Add the smashed garlic and cook for 30 seconds. Pour in the adobo sauce and scrape up any browned bits from the bottom.
Return the chicken thighs to the pot skin-side up. Add the bay leaves and whole black peppercorns.
Lock the lid, set the valve to sealing, and cook on high pressure for 12 minutes. Allow a natural pressure release for 10 minutes, then quick release the remaining pressure.
Remove the chicken thighs to a plate. Discard the bay leaves. Set the Instant Pot to saute mode and let the sauce simmer for 5 to 7 minutes until reduced and slightly thickened.
Serve the chicken thighs over steamed rice and spoon the reduced adobo sauce generously over the top. Garnish with sliced green onions.