Barbecue Beefwiches

Ingredients 1 (3 pound) lean beef rump roast Vegetable cooking spray 1 1/2 cups reduced-calorie catsup 1/4 cup plus 2 tablespoons red wine vinegar 1/3 cup firmly packed dark brown sugar 1 tablespoon dried onion flakes 1 teaspoon liquid smoke 1/2 teaspoon salt 1/2 teaspoon pepper 1/8 teaspoon garlic powder 2 1/2 cups finely shredded