I a 5- to 6-quart slow-cooker, combine onion, garlic, tomato paste and oregano. Season roast with salt and pepper and place on top of onion mixture. Add 2 cups water, cover and cook on high until meat is tender and can easily be pulled apart with a fork, about 4 1/2 hours (or 9 hours on low). Let cool 10 minutes, then shred meat in slow cooker with 2 forks and stir in vinegar to taste.
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