Grind onion, ginger, garlic, mustard seeds, cumin seeds, cinnamon stick, peppercorns, and cloves with just enough vinegar to keep the mixture moving in a blender or food processor until you have a thick paste.
Mix remaining vinegar and water together in a large bowl. Add pork cubes to the bowl and thoroughly rinse the meat with the vinegar mixture; drain. Pat pork dry with paper towel.
Mix the paste and pork cubes together in a clean bowl, assuring the pork cubes are coated in paste. Cover bowl with plastic wrap. Marinate in refrigerator 8 hours to overnight.
Heat vegetable oil in a large skillet over medium-high heat. Fry curry leaves in hot oil until golden brown, 2 to 3 minutes. Add marinated pork, tomatoes, and turmeric; cook and stir until the tomatoes are completely softened, about 10 minutes.
Season the pork mixture with salt. Pour enough pork stock over the mixture to assure everything is at least half-submerged. Bring the stock to a simmer, place a cover on the skillet, reduce heat to medium-low, and simmer until the pork is completely tender, 1 to 2 hours.
Remove lid from skillet and continue to simmer the mixture until the sauce is very thick, about 10 minutes. Garnish with cilantro.