Step 1: In a large mixing bowl, beat together the softened cream cheese, sweetened condensed milk, freshly squeezed key lime juice, key lime zest, and vanilla extract until smooth and well combined.
Step 2: In a separate, chilled bowl, beat the 1 cup heavy cream and powdered sugar together with an electric mixer on high speed until stiff peaks form.
Step 3: Gently fold about half of the whipped cream from Step 2 into the key lime cream mixture from Step 1, being careful not to deflate the whipped cream.
Step 4: In a medium bowl, combine the crushed graham crackers with the melted butter, stirring until the crumbs are evenly moistened.
Step 5: To assemble, layer about one-third of the graham cracker mixture at the bottom of a large, clear trifle bowl. Spoon about one-third of the key lime cream mixture over the graham crackers. Repeat these layers two more times, finishing with a layer of key lime cream on top.
Step 6: Spread the remaining fresh whipped cream over the top layer of key lime cream. Garnish with fresh lime slices and extra key lime zest. Cover and refrigerate for at least 2 hours (preferably 4-6 hours) to allow the flavors to meld and the trifle to set before serving.