In a saucepan, combine brown sugar and cornstarch. Drain peaches, reserving 1/2 cup liquid. Set peaches aside. Stir reserved liquid into brown sugar mixture; bring to a boil. Cook and stir for 2 minutes. Add rhubarb; simmer for 8 minutes. Stir in peaches and vanilla. Pour into an ungreased 8-in round baking pan.
Dip one side of biscuits in sugar; place over hot fruit with sugar side up. Bake, uncovere, at 375 degrees for 20-24 minutes or until biscuits are golden brown. Serve warm.