Trim excess fat from meat. Place roast in a slow cooker, cutting it in half if needed to fit. Add celery and onion. In a medium bowl, stir soup, vinegar, bouillon, sage and pepper together. Pour into crockpot. Cover and cook HIGH for 4.5 to 5 hours or on LOW for 9 to 10 hours or until meat is very tender.
In a small bowl, stir water and flour until smooth. Stir into boiling liquid around roast. Cover and cook on HIGH for 20 to 30 minutes until thickened. Transfer meat to a warm platter. Serve gravy separately. Serve with mashed potatoes, rice or noodles.