Make the crust: Combine the graham cracker crumbs, melted butter, and granulated sugar until evenly moistened. Press firmly into the bottom and sides of a 9-inch pie dish.
Chill the crust: Place the crust in the refrigerator while preparing the filling.
Beat the cream cheese: Add softened cream cheese and granulated sugar to a mixing bowl. Beat until completely smooth and creamy.
Add the lime: Mix in the key lime juice, lime zest, and vanilla extract until well combined.
Whip the cream: In a separate bowl, beat the cold heavy cream and powdered sugar until soft to medium peaks form.
Fold the mixtures together: Gently fold the whipped cream into the lime and cream cheese mixture until smooth and airy. Avoid overmixing.
Fill the crust: Spoon the mousse into the chilled graham cracker crust and smooth the top with a spatula.
Chill: Cover and refrigerate for at least 4 hours, or preferably overnight, until the filling is firm.
Make the topping: Beat the cold heavy cream, powdered sugar, and vanilla until soft peaks form.
Decorate and serve: Add the whipped cream to the chilled pie and finish with lime zest or thin lime slices. Slice with a sharp knife and serve cold.
