Add mushrooms to hot drippings in skillet; cook, stirring often, until beginning to brown, about 4 minutes. Add carrots and leek; cook about 3 minutes. Add 2 tablespoons of the reserved flour; cook, stirring constantly, about 1 minute. Add vegetable stock and white wine vinegar; cook, stirring often, until sauce thickens slightly, about 6 minutes. Stir in cream and season with more salt and pepper; add chicken, nestling chicken into sauce mixture.