Heat oil in heavy large skillet over medium heat. Add mushrooms and lemon juice and saute until mushrooms are brown, about 6 minutes. Add dry vermouth. Boil 3 minutes, scraping up browned bits. Set aside. (Can be prepared 1 day ahead. Cover and chill.) Blend ricotta, yolks and 1 tablespoon Parmesan in processor. Add flour and process until smooth. Transfer mixture to large bowl. Season with salt and pepper. In medium bowl, beat egg whites until stiff but not dry. Fold whites into ricotta mixture in 2 additions. Fold in breadcrumbs. Pour water into heavy large skillet to depth of 2 inches. Bring to simmer over medium heat. Drop batter by rounded tablespoons into water. Cover and cook 3 minutes. Turn dumplings over, cover pan and cook 3 minutes longer. Using slotted spoon, transfer dumplings to paper towels and drain. Bring mushroom sauce to simmer. Add 6 tablespoons butter and stir just until melted. Place dumplings on platter and spoon sauce over. Sprinkle with additional Parmesan and chives.