Cut stems and center ribs away form kale leaves and coarsely tear leaves. In a large heavy saucepan of boiling salted water cook leaves 5 minutes and drain well in a colander. In pan cook bacon over moderate heat, stirring, until crisp. Pour off all but about 1 tablespoon fat from pan. Add kale to bacon and saute over moderately high heat, stirring, until heated through. Toss kale with vinegar and season with salt and pepper.