In a mixing bowl, combine graham cracker crumbs, granulated sugar, and melted butter. Mix until fully combined and the texture resembles wet sand.
Press the mixture firmly into the bottom and up the sides of a 9-inch pie dish to form the crust. Chill the crust in the refrigerator for about 30 minutes to set.
In a separate bowl, whisk together the sweetened condensed milk, fresh lime juice, lime zest, and vanilla extract until smooth and well-blended.
In another bowl, whip the heavy cream and powdered sugar to stiff peaks using an electric mixer.
Gently fold the whipped cream into the lime mixture until combined, being careful not to deflate the cream.
Pour the key lime filling into the chilled crust, smoothing the top with a spatula.
Cover the pie with plastic wrap and refrigerate for at least 4 hours or overnight for best results to allow the filling to firm up.
Once set, slice the pie and serve chilled, optionally garnished with lime slices or a dollop of whipped cream.