Barbecue Beef Sandwiches

3-4 pounds beef roast or round steak
2 tsp vinegar
14 oz ketchup
1 cup water
2 Tbsp Worcestershire sauce
3 stalks celery, chopped
3 small onions, chopped
1 Tbsp salt
1 tsp chili powder

Mix all ingredients and pour over meat. Cover and bake at 250F for 5 hours. Break meat up as it absorbs juice. Serve on Kaiser rolls.