Prepare the Syrup: Mix the water, sugar, lemon zest, and Grand Marnier in a small bowl until the sugar dissolves. Set aside.
Make the Topping: In a medium bowl, combine the mascarpone and Grand Marnier, stirring until smooth. In another bowl, whip the heavy cream with sugar and vanilla until medium-stiff peaks form. Gently fold the whipped cream into the mascarpone mixture in thirds, ensuring it is fully incorporated. Set aside.
Assemble the First Layer: Quickly dip the ladyfingers into the syrup, ensuring both sides are coated without over-soaking. Arrange the dipped ladyfingers in a single layer in the 9 x 13-inch dish.
Add the Cream and Berries: Spread half of the mascarpone-cream topping over the ladyfingers, smoothing it evenly. Sprinkle one-third of the fresh berries over the cream. If using larger berries, press them gently into the cream to create an even surface.
Repeat Layers: Dip and layer the remaining ladyfingers, followed by the rest of the mascarpone-cream topping. Spread evenly to the edges of the dish. Cover with plastic wrap and refrigerate for 8 hours or overnight.
Decorate and Serve: Before serving, arrange the remaining berries on top. Create a patriotic flag design or scatter them evenly. Serve chilled.