Cook green beans in 1-inch simmering water, covered, for 6 minutes. Drain and set aside. Return skillet to stove and set burner to medium high. Add bacon and brown. When fat begins to render, add chopped onions to the pan and cook until tender with the bacon. Add cooked green beans to the pan and turn to coat in bacon drippings and onions. When the beans are hot and bacon crisp at edges and onions translucent add vinegar to the pan and season with beans with sugar. Allow the vinegar to evaporate and the sugar to combine with pan drippings, 1 to 2 minutes, then serve.