Spray 4-quart slow cooker with cooking spray. In slow cooker, mix onions, vinegar and butter. Cover; cook on Low heat setting 8 hours or until onions are golden brown.
In large bowl, beat cream cheese with electric mixer on medium speed until creamy. Add whipping cream; beat on low speed until smooth. Stir in blue cheese, salt and pepper until well blended. Add cream cheese mixture to onion mixture in slow cooker; stir until blended.
Cover; cook 20 minutes longer or until thoroughly heated. Sprinkle with bacon and parsley.