Boil Port and wine in heavy small saucepan until mixture is reduced to 2/3 cup, about 8 minutes. Set aside.
Sprinkle steaks with salt and pepper. Dust with flour; shake off excess. Melt 1 tablespoon butter with oil in heavy large skillet over medium-high heat. Add steaks; cook to desired doneness, about 4 minutes per side for medium-rare. Transfer steaks to platter (do not clean skillet); tent steaks with foil to keep warm.
Add Port mixture and vinegar to same skillet and bring to boil, scraping up any browned bits. Boil until reduced to sauce consistency, about 2 minutes. Remove from heat. Add remaining 2 tablespoons butter and whisk just until melted. Season sauce to taste with salt and pepper. Transfer steaks to plates. Spoon sauce over and serve.