Drizzle the vegetable oil into a wide pot over medium heat.
Once heated, add the pork belly (in batches if the pot is not wide enough to fit them in a single layer) and cook, turning occasionally, until golden brown on each side.
Remove the pork to a plate and set aside. Discard all but a thin layer of oil.
Add the onion to the pot and cook, stirring occasionally, until softened. Stir in the garlic and cook just until fragrant, 30 seconds to 1 minute.
Add the browned pork back to the pot, then pour in the vinegar, soy sauce, and water. Pour in a little more water if it is not enough to just cover the pork.
Stir in the brown sugar, peppercorns, and bay leaves.
Once bubbling, reduce heat to a low simmer, partly cover the pot, and cook, stirring occasionally and skimming any excess oil off the top, until the liquid has reduced by about half and the pork is tender, 1-2 hours. Adjust seasonings to taste.
Serve with freshly steamed jasmine rice and optionally a sprinkle of fried garlic.