Place beans in a large saucepan and cover with water; bring to a boil. Cook, uncovered, for 8-10 minutes or until crisp-tender.
Meanwhile, in another saucepan, saute onion in butter. Stir in flour and sugar until blended; gradually add milk. Bring to a boil; cook and stir for 2 minutes or until thickened. Reduce heat to low; stir in the sour cream, parsley and vinegar until blended (do not boil).
Drain beans; place in a serving bowl. Season with salt and pepper. Top with sauce and bacon.