In a large mixing bowl, whip the heavy cream, powdered sugar, and vanilla extract together until soft peaks form.
On a rectangular serving platter, arrange a layer of graham crackers to cover the bottom.
Spread a layer of the whipped cream mixture over the graham crackers, then add a layer of sliced strawberries and blueberries.
Repeat the layers, alternating between graham crackers, whipped cream, and berries until all ingredients are used, finishing with a layer of whipped cream on top.
Cover the cake with plastic wrap and refrigerate for at least 4 hours, or overnight for best results, to allow the graham crackers to soften.
Before serving, garnish with additional berries if desired and slice into squares.