In a large deep skillet, cook the pancetta over moderate heat until it is slightly crisp and the fat is rendered, 8 to 10 minutes. Stir in the shallots, season with salt and pepper and cook until golden, about 10 minutes. Add the brussels sprouts and vinegar and bring to a boil. Reduce the heat to low, cover the pan and cook until the sprouts and shallots are tender and the pan is almost dry, 10 to 12 minutes. Transfer to a bowl and keep warm until serving.