Preheat grill to 450˚F.
For vinegar bbq sauce: Place all ingredients into a mixing bowl and whisk together until smooth and completely blended. Set aside until ready to use.
Place pork belly on a clean cutting board, skin-side up and score the skin. Pat dry with a paper towel and flip over so the meat-side is up.
Place all dry rub ingredients into a small bowl and whisk together. Pour rub over meat-side of belly only and rub over entire surface.
Place onions and one bottle of beer into a roasting dish, in an even layer, and top with pork (meat/rub side down).
Place roasting dish into the hot grill and cover with the grill lid. Cook for about 45 minutes.
Lower heat to 250˚F and continue to cook for about 1 hour. Remove lid and add remaining beer. Re-cover and cook for an additional 2 hours.
At this point the fat should have rendered and the skin should be extra crispy.
Carefully remove the pork belly from the baking dish and transfer onto a cutting board. Allow the slab to rest for about 10 minutes.
Chop into pieces and serve atop sliced white bread with a generous drizzle of the vinegar bbq sauce.