Cook bacon in heavy large skillet over medium-high heat until crisp. Transfer bacon to paper towel to drain. Discard all but 2 tablespoons fat from pan. Crumble bacon.
Combine flour, salt, pepper and thyme in shallow dish. Add liver and turn to coat; shake off excess. Heat fat in skillet over medium-high heat. Add liver and cook until tender and brown on both sides, about 5 minutes. Transfer liver to plate; cover with foil and keep warm. Add onion to skillet and saute until golden, about 5 minutes. Mix in broth, Port and vinegar. Boil mixture until reduced to sauce, about 2 minutes. Season to taste with salt and pepper. Pour sauce over liver. Sprinkle with bacon and serve.