1Place onion slices and broth in 3 1/2- to 4-quart slow cooker. Brush all surfaces of beef roast with vinegar. Place on onions. Sprinkle with dressing mix (dry), salt and pepper.
2Cover and cook on low heat setting 8 to 10 hours or until beef is tender.
3About 5 minutes before serving, remove beef from cooker; place on cutting board. Cut beef across grain into thin slices; return slices to cooker and mix well.
4Serve beef mixture on buns with bell pepper and cheese. Beef mixture will hold on low heat setting up to 4 hours.