Preheat your oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.

In a large mixing bowl, combine the cake mix, water, vegetable oil, egg whites, and vanilla extract. Mix until smooth.

Pour the batter into the prepared baking pan and spread it evenly. Bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean.

Once baked, remove the cake from the oven and let it cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.

Once the cake is completely cool, spread a layer of Cool Whip over the top of the cake evenly.

Using the blueberries, create a rectangle in the top left corner of the cake to represent the stars of the flag.

Next, arrange the sliced strawberries in rows to create the red stripes below the blueberries. Leave some space between the rows to represent the white stripes.

Fill in the remaining space with Cool Whip for the white stripes of the flag.

If desired, sprinkle red, white, and blue sprinkles on top for an extra festive touch.

Cover the cake with plastic wrap and refrigerate for at least 2 hours, or overnight, to allow the flavors to meld and for easy serving.