Spray 3- to 4-quart slow cooker with cooking spray. In cooker, place onion; add broth. Brush all surfaces of beef roast with vinegar. Place on onions. Sprinkle with dressing mix, salt and pepper.
Cover; cook on Low heat setting 8 to 10 hours.
Remove beef from cooker; place on cutting board. Cut beef across grain into thin slices. Return slices to cooker; mix well.
Spoon beef onto bottoms halves of buns. Top each with bell pepper, cheese and top half of bun. Serve sandwiches with juices from slow cooker.