Begin by preparing your marinade. In a large bowl, mix calamansi juice, fish sauce, minced garlic, chopped Thai chili peppers, brown sugar, salt, and pepper until the sugar completely dissolves.
Place your sliced pork belly in the marinade and gently massage the meat to coat evenly. Transfer everything to a container or zip-lock bag. Let it marinate in the refrigerator for 4-8 hours, turning occasionally. Don't marinate longer than 8 hours as the meat can become mushy.
Take the meat out of the refrigerator 30 minutes before cooking. Set aside 1 cup of the marinade for basting. Heat your grill to medium-high heat (375°F/190°C). While the grill heats up, boil the reserved marinade with oyster sauce and oil in a small pan for 3-5 minutes until slightly thickened. This is your basting sauce.
Oil your grill grates well. Place the marinated pork belly on the hot grill and cook for 2-3 minutes on each side until you see nice grill marks. When the meat starts losing its pink color, begin brushing it with your basting sauce. Keep grilling for another 4-5 minutes per side, occasionally basting, until the pork is fully cooked and reaches 160°F inside.
Transfer the grilled pork to a plate and let it rest for 5 minutes to keep the juices in. While waiting, make your dipping sauce by mixing vinegar, minced garlic, chopped onions, chopped chilies, salt, and pepper in a small bowl.
Cut the meat into serving pieces and arrange on a platter lined with banana leaves. Serve hot with your spiced vinegar dipping sauce and steaming white rice.