Place potatoes in a slow cooker. Trim fat from roast; brown in hot oil on all sides in a skillet. Place meat in slow cooker. Discard all but 1 tbsp drippings from skillet; saute onion until tender. Stir in tomato sauce, brown sugar, Worcestershire sauce, vinegar, and salt. Pour over meat and potatoes. Cover and cook on high for 4-5 hours or until the meat is tender. Before serving, pour sauce into a skillet. Cook and stir over medium-high heat until thickened; serve with potatoes and meat.