Melt butter in a large skillet. When it is hot, stir in onion, cabbage, sugar, salt, and pepper. Cook over very low heat, stirring occasionally, for 15 to 20 minutes, or until onion and cabbage are softened.
While onion and cabbage are cooking, cook noodles in lightly salted, boiling water as package directs. Drain in a colander. Sprinkle cabbage with a few drops of vinegar, or to taste, and toss to combine. Add noodles to skillet, tossing to blend. Continue to cook, until heated through.