Pierce beef roast and place it into a slow cooker with bell peppers, onions, and garlic. Stir in tomato paste and vinegar. Sprinkle with garlic powder, seasoned salt, salt, and black pepper.
Cover and cook on Low until beef is tender, 7 to 8 hours. Remove beef and shred into long strands using 2 forks. Return shredded meat to the slow cooker; mix until combined.
Cover and continue cooking on Low until meat and vegetables are very tender, about 30 minutes.