Place corned beef in a 6-quart heavy pot and pour in enough water to cover beef by 2 inches. Bring water to a boil, skimming liquid occasionally to remove fat from top. Reduce heat to medium-low, cover with lid, and simmer for 2 hours.
Place cabbage, potatoes, onions, and carrots around beef and return to a boil. Lower heat to medium-low, cover, and simmer until vegetables are tender, about 40 minutes. Transfer beef to a serving platter, reserving 1 1/2 cups cooking liquid. Surround meat with the vegetables.
For Sauce: Melt butter in a saucepan over medium heat. Whisk flour into melted butter until dissolved and bubbling, about 1 minute. Add cooking liquid from beef and vinegar to butter-flour mixture; cook, stirring frequently, until sauce is slightly thickened, about 4 minutes. Remove saucepan from heat and whisk sour cream and mustard into sauce. Serve sauce alongside corned beef and vegetables.