Combine the soy sauce, garlic, sugar and peppercorns in a large bowl. Add the pork and marinate in the refrigerator for at least 2 hours or overnight, covered (can be marinated in a resealable 1-gallon freezer bag). Mix twice at regular intervals to marinate thoroughly.
Drain the pork in a colander over a bowl; reserve the marinade and garlic. Heat the oil in a large Dutch oven over medium heat. Brown the pork in batches, making sure not to crowd the meat and turning often until all sides are brown, 6 to 8 minutes per batch (the peppercorns may adhere to the pork, which is fine). Watch closely: The sugar in the marinade will cause the pork to darken quickly if the pot is too hot-lower the heat if necessary to avoid burning. Remove the pork with a slotted spoon; set aside.
Discard all but 2 tablespoons of the oil from the Dutch oven. Add the garlic from the marinade and the onions, and cook, stirring, until the onions are translucent, 10 to 12 minutes.
Add back the pork, the strained marinade, 1 cup water and the bay leaves, and bring to a boil. Lower the heat to medium, cover and cook at a medium boil, stirring periodically, until the pork is tender but not falling apart, about 1 hour and 25 minutes. The cooking liquid will be reduced by at least 2/3.
Add the vinegar, but do not stir. Cook, uncovered, until the sauce is reduced to the consistency of a loose marinara, about 20 minutes more. Remove from the heat, and skim and discard fat (pork belly will render a good amount). Allow the pork and sauce to sit 15 minutes before serving; the sauce will continue to thicken. Serve over jasmine rice.
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