Preheat oven to 400°.
Combine first 4 ingredients in a large bowl. Add peaches, blueberries, lemon juice, and vanilla, and toss gently. Spoon mixture into an 8-inch square baking dish coated with cooking spray. Bake mixture at 400° for 15 minutes.
Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour and next 4 ingredients (flour through salt)in a medium bowl; cut in margarine with a pastry blender or 2 knives until mixture resembles coarse meal. Add buttermilk; stir just until flour mixture is moist.
Turn dough out onto a lightly floured surface; knead lightly 3 times. Roll dough to about a 1/4-inch thickness; cut into 16 biscuits using a 2-inch biscuit cutter.
Remove dish from oven; arrange biscuits on top of hot fruit mixture. Bake an additional 20 minutes or until biscuits are golden. Garnish with mint sprigs, if desired.