Preheat the oven to 350°F. Lightly coat two 9x2-inch round cake pans with the baking spray, and line the bottoms with parchment paper.
In a medium bowl, combine the flour, cocoa powder, baking soda, and salt. Reserve. In a medium bowl, stir together the buttermilk, food coloring, 2 tsp. vanilla, and the vinegar. Reserve.
In a large bowl, beat the butter and 2 cups sugar with an electric mixer on medium speed until very light and fluffy, about 3 minutes. Add the eggs, one at a time, beating between additions until combined. Alternate adding the dry flour mixture and the liquid buttermilk mixture to the batter, with mixer on low speed, scraping down the sides occasionally. Do not over mix.
Divide the batter evenly between the 2 prepared pans. Bake, on the same rack, about 35 minutes or until a toothpick inserted in the center comes out clean. Let cool in pans on wire racks for 10 minutes. Remove from pans and remove parchment. Let cool completely on wire racks. Cake layers may be made up to 2 days ahead, and left at room temperature lightly covered or wrapped in plastic wrap and frozen up to one week ahead of trifle preparation.
In a large bowl beat the cream cheese and 3/4 cup sugar with an electric mixer on medium speed until creamy and smooth, about 3 minutes. Beat in the 2 tsp. vanilla
With mixer on low speed, slowly pour in the cold cream. Increase speed to high and mix until voluminous and fluffy, 1 to 2 minutes. Filling may be made 3 to 4 hours ahead of assembly and kept, covered and refrigerated.
If necessary, cut one cake layer into pieces to fit snugly in the bottom of a 4-quart trifle dish or glass bowl. Spread half the filling over the cake layer in the dish. Layer the blueberries over the filling, reserving a handful for the top of the trifle.
If necessary, cut the second cake layer to fit snugly over the blueberries. Press down gently and the blueberries and filling will move out slightly to the edge of the dish. Spread remaining filling on top of second layer. Cover the trifle with a plastic lid or plastic wrap. Pack raspberries, strawberries, and remaining blueberries in a zip-top bag or other storage containers. The trifle and berries may be refrigerated at this point for up to 24 hours. If chilled, let stand at room temperature for 30 minutes prior to serving.
At the picnic, top with the reserved raspberries, strawberries, and blueberries.