Cook potatoes in boiling, salted water till tender; drain, peel, and dice (about 6 cups].
Keep potatoes warm.
In skillet cook beef, onion, and green pepper in margarine till vegetables are tender but not brown.
Blend in soup, milk, vinegar, and salt.
Gently stir in warm potatoes.
Sprinkle cheese atop potato mixture; cover and heat just till cheese begins to melt, about 3 or 4 minutes.