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Preheat oven to 350°F. Prepare cake mix as directed on package, adding vanilla. Remove 1 1/2 cups of the batter. Tint blue with neon blue food color. Tint remaining batter red with cocoa powder and red food color. Pour each color batter into separate 9x5-inch loaf pans sprayed with no stick cooking spray.
Bake 20 to 25 minutes for blue-tinted cake; 30 to 35 minutes for red-tinted cake. Cool in pans 10 minutes. Remove from pans; cool completely.
Trim cakes to remove rounded tops and edges. Slice red cake in half horizontally to form 2 thin layers. Place 1 of the layers on platter. Slice remaining red cake layer in half lengthwise. Slice blue cake in half lengthwise. (Blue and red cakes should be the same dimensions.)
Frost red cake layer on platter with 1/3 of the frosting. Top with a layer of 1 red and 1 blue cake placed side by side. Frost with 1/3 of the frosting. Repeat cake layer and frosting. Garnish with raspberries and blueberries to resemble a flag.