1. Bring 2 cups water to a boil in a saucepan. Stir in mushrooms; cover, remove from heat, and let stand 10 minutes.
2. Uncover pan; bring mushroom mixture to a boil. Stir in bulgur; cover, remove from heat, and let stand 30 minutes or until liquid is absorbed. Spoon bulgur mixture into a large bowl.
3. Heat butter and oil in a large nonstick skillet over medium-high heat. Add onion, celery, carrot, and garlic to pan; sauté 7 minutes or until tender and lightly browned. Add vegetables to bulgur mixture; cool slightly. Stir in salt and pepper. Remove casings from sausage. Crumble sausage into bulgur mixture; stir well to combine.
4. Add water to a large Dutch oven to a depth of 2 inches; set a large vegetable steamer in pan. Bring water to a boil over medium-high heat. Add cabbage leaves to steamer. Steam cabbage, covered, 6 minutes or until tender and pliable. Remove cabbage from steamer (do not drain water). Rinse cabbage with cold water; drain and pat dry.
5. Working with one cabbage leaf at a time, place 1/2 cup bulgur mixture in center of leaf. Fold in edges of leaf; roll up. Repeat procedure with the remaining cabbage leaves and bulgur mixture to form 12 cabbage rolls. Stack rolls evenly in steamer.
6. Return Dutch oven to medium-high heat; bring water to a boil. Steam rolls, covered, 30 minutes, adding more water if necessary.
7. Combine tomatoes, red wine vinegar, and sugar in saucepan; cook over medium heat 5 minutes or until thoroughly heated, stirring occasionally. Remove from heat; stir in parsley. Serve sauce with rolls.