Hold the tail tube portion of the squid and with fingers and pull the cuttlebone (the thin, clear cartilage inside the tube) from inside the body, leaving the ink sack intact.
Wash squid thoroughly and drain well.
In a pot over medium heat, combine squid, vinegar, 2 cloves of the garlic, peppercorns, and bay leaf. Bring to a boil for about 3 to 5 minutes or just until squid changes color.
Remove from heat and drain, reserving about 1 cup of the liquid.
In a pan over medium heat, heat 1 tablespoon of the oil. Add onions and the remaining 2 cloves garlic and cook until softened.
Add tomatoes and cook, mashing with the back of a spoon, until softened.
Add fish sauce and cook for about 1 to 2 minutes.
Add squid and cook, stirring regularly, for about 1 minute.
Add reserved liquid and continue to cook until squid is cooked through and sauce is reduced.
Add the remaining tablespoon of oil and toss. Season with salt and pepper to taste. Serve hot.