In 12-inch skillet, melt butter over medium-low heat. Cook onions in butter 15 to 20 minutes, stirring frequently, until very soft and light golden brown. Add vinegar; cook 5 minutes, stirring frequently. Remove from heat; cool completely, about 20 minutes.
In medium bowl, mix sour cream, cream cheese, 2 tablespoons chives, the salt and pepper. Stir in caramelized onions. Serve immediately or refrigerate until serving time. Garnish with additional chives.