With a small knife, poke surface of the skin all over without piercing the meat.
Pour the vinegar, massaging onto the belly. Season with salt and pepper.
With a knife, cut the thinner top portion of the lemongrass stalk and the very woody base. Peel off the tougher outer layers to get to the tender part of the stalk. With the back of a knife, pound the stalk a few times.
Place the pork belly skin side down on a flat working surface. On the longer side of the belly, arrange the lemongrass stalks, onions, and garlic horizontally on the lower part of the belly.
Roll pork belly into a tight log, ending with skin side up. Using kitchen twine, tie on three to four places to secure shape.
Refrigerate, uncovered, overnight to chill.
Using paper towels, pat down surfaces of meat to completely dry. Place the belly roll on a roasting pan fitted with a rack.
Bake in a preheated 180 F oven for about 3 1/2 hours.
Increase temperature to 320 F and continue to cook for about 1 1/2 to 2 hours or until meat is cooked through and browns.
Increase temperature to 430 F and continue to cook for about 30 to 40 minutes or until skin is very crisp.
Remove from oven and let rest for about 10 to 15 minutes before slicing.