Mix ½ cup vinegar with pig's blood. Set aside.

In a large pot over medium heat, sauté the chopped onion and minced garlic in oil until they become tender and aromatic.

Add the pork belly to the pot and cook until the fat is rendered. Stir in fish sauce, bay leaves, and season with salt and pepper.

Pour in the vinegar and let it simmer for 2 minutes without stirring.

Stir in the blood and vinegar mixture continuously to prevent clumping. Simmer for an additional 5 minutes.

Pour in the water and add the green long chili peppers. Bring the mixture to a boil, then cover the pot with a lid, lower the heat to low, and cook until the pork is tender.

Once the pork is tender, remove the lid and let the stew simmer for a few minutes to reach the desired sauce consistency.

Turn off the heat and transfer the stew into serving bowls.