In a medium bowl, combine graham cracker crumbs and melted butter. Press the mixture firmly into the bottom of a 9x13 inch baking dish to form the crust.
In another bowl, beat together the softened cream cheese, powdered sugar, and vanilla extract until smooth. Gently fold in 1 cup of whipped topping until well combined. Spread this cream cheese mixture evenly over the graham cracker crust.
In a separate bowl, dissolve the Jell-O powder in 1 cup of boiling water, stirring until fully dissolved. Add 1 cup of cold water and mix well. Allow the mixture to cool slightly.
Layer the mixed berries evenly over the cream cheese layer. Pour the cooled Jell-O mixture over the berries, ensuring they are well covered.
Refrigerate for at least 4 hours, or until the Jell-O is fully set.
Before serving, spread the remaining 1 cup of whipped topping over the Jell-O layer for a finishing touch. Cut into squares and serve chilled.