Heat the oil in a large, deep skillet. Add the pancetta, garlic and rosemary and cook over moderately high heat until the garlic is softened, about 2 minutes. Add the chard stems and cook, stirring occasionally, until crisp-tender and lightly browned, about 7 minutes. Add the chard leaves, season with salt and pepper and cook, tossing, until wilted, about 5 minutes. Stir in the vinegar, toss to coat and serve.