In a bowl, mix together the soy sauce, vinegar, calamansi juice, salt, black pepper, garlic powder, onion powder, paprika, oregano, and sugar if using.
Rub the marinade all over the pig, inside and out, ensuring it is well-coated.
Stuff the cavity of the pig with chopped lemongrass, crushed garlic, quartered onion, and bay leaves.
Cover and refrigerate for at least 12 hours, preferably overnight, to allow the flavors to penetrate the meat.
Preheat your oven to 375째F (190째C) if using an oven. Alternatively, prepare a large outdoor grill for indirect heat.
If using an oven, place the pig on a rack in a large roasting pan. If grilling, secure the pig on a rotisserie or spit.
Roast the pig for about 4-6 hours, turning occasionally, until the skin is crispy and golden brown and the meat is tender. Baste every 30 minutes with a mixture of soy sauce and melted butter.
Use a meat thermometer to ensure the internal temperature of the meat reaches at least 165째F (74째C).
Once cooked, let the lechon rest for 20-30 minutes before carving.
Enjoy your meal!