Heat oven to 425°F. Unroll pie crust on cutting board. Using sharp knife, cut out 2 large ovals from pie crust. Gather up dough scraps into ball and roll out with rolling pin; cut third oval. Place on ungreased large cookie sheet. Bake 5 minutes.
Meanwhile, stir together grape halves, walnuts, rosemary and vinegar.
Remove flatbreads from oven. Arrange Brie slices across each flatbread, leaving 1-inch border around edges. Spoon grape mixture over Brie.
Bake 5 to 8 minutes longer or until grapes are tender and flatbread is golden brown. Remove from cookie sheet to cooling rack. Cool 5 minutes. Cut into wedges; serve warm.