Line a 9x5 inch standard loaf pan with plastic wrap with overhang over the edges to easily lift the cake out of the pan.
Make the whipped cream first. Using a hand mixer or a stand mixer with a whisk attachment, beat the heavy cream, powdered sugar and vanilla on medium-high speed until soft peaks form, about 3 minutes. Be careful not to over beat.
Once whipped, layer the cake:Spread a few Tablespoons of whipped cream into a thin layer on the bottom of the prepared pan. This will help the bottom layer of graham crackers stick.
Layer graham crackers on top to make an even layer. It's okay if it's not completely covered and there are cracks.
Spread a approximately ¾ cup of whipped cream on top of the graham crackers.
Then layer 1/2 cup chopped strawberries and 1/2 a cup of blueberries, another ¾ cup whipped cream, another layer of graham crackers, ¾ cup whipped cream, another layer 1/2 cup chopped strawberries and 1/2 a cup of blueberries, ¾ cup whipped cream, graham crackers, then the rest of the whipped cream and the rest of the berries.
Cover the icebox cake with plastic wrap and place in the freezer for at least 4 hours or overnight to set.
Allow to soften at room temperature for 10-15 minutes then use a warm knife to cut into slices.