Cook the beans until crisp-tender. Wrap about 10 beans in each bacon strip; secure with a toothpick. Place on a foil-covered baking sheet. Bake at 400° for 10-15 minutes or until bacon is done.
In a skillet, saute onion in butter until tender. Add vinegar, sugar and salt; heat through. Remove bundles to a serving bowl or platter; pour sauce over and serve immediately.