In a microwave-safe bowl, combine brown sugar, butter and corn syrup. Microwave, uncovered, on high for 2-1/2 minutes or until sugar is dissolved and corn syrup is bubbly, stirring frequently. Stir in vanilla and baking soda; microwave 20 seconds longer or until the mixture begins to foam.
In a large bowl, combine the cereals. Add syrup mixture and toss to coat. Transfer to two greased 15x10x1-in. baking pans. Bake, uncovered, at 300° for 30 minutes, stirring every 10 minutes. Remove to waxed paper to cool. Store in an airtight container.