Heat large pot of water to boiling. Salt water, add Brussels sprouts and cook just until tender, about 8 minutes Or follow directions on package for frozen sprouts.
Drain and cool sprouts under running water. Cut each in half through the stem end.
Heat large heavy skillet over medium high heat. Add bacon and fry until golden brown. Transfer to paper towels to drain. Pour off all but 3 tablespoons fat.
Add onions and 1 tablespoon butter to skillet; cook over medium heatuntil softened, about five minutes. Remove onions with slotted spoon. Pour off fat and wipe out skillet.
Add remaining 5 tablespoons butter to skillet and heat over medium heat. Add sprouts and cook until lightly browned, about ten minutes. Add bacon and onions. Season to taste with salt and pepper. Sprinkle with vinegar and heat through. Serve hot.