1. To prepare galette, weigh or lightly spoon flours into dry measuring cups; level with a knife. Combine flours, granulated sugar, and salt in a medium bowl, and cut in butter with a pastry blender or 2 knives until mixture resembles coarse meal. Add ice water; stir just until moist. Pat dough into a 7-inch circle on plastic wrap; cover. Chill for 15 minutes.
2. Combine plums, 1/2 cup brown sugar, 1/4 cup Armagnac, and vanilla bean in a large skillet over medium heat, and cook for 10 minutes or until plums are tender, stirring occasionally. Cool to room temperature.
3. Preheat oven to 300°.
4. Tear bread into 1-inch pieces. Place on baking sheet; bake 30 minutes or until dry and golden. Place bread in a food processor; process until coarse crumbs measure 1/4 cup.
5. Increase oven to 425°.
6. Unwrap and place dough on a baking sheet. Roll dough into a 15-inch circle; sprinkle dough with breadcrumbs, leaving a 2-inch border. Arrange plum mixture over crumbs. Fold edges of dough over plum mixture (dough will only partially cover plum mixture). Brush dough edges and top of fruit with melted butter; sprinkle with remaining 1 tablespoon brown sugar. Bake at 425° for 15 minutes. Reduce oven temperature to 375° (do not remove galette from oven); bake an additional 20 minutes or until bubbly and edges are golden. Cool 5 minutes on pan; loosen galette from pan. Cool an additional 30 minutes on pan.
7. To prepare cream, combine sour cream and remaining ingredients in a small bowl, stirring with a whisk. Serve over galette slices.