This recipe has been adapted from Al Roker's book, "Big Bad Book of Barbecue" (Scribner; 2002)
Position rack in center of oven; preheat to 350 degrees.
In a medium bowl, mix together cracker crumbs and butter. Press into a 9-inch pie plate. Bake until crust begins to puff, about 8 minutes. Remove from oven; transfer to a cooling rack. If crust has pulled away from sides, use the back of a spoon to press it back. Let cool completely.
In a medium bowl, stir together condensed milk, zest, juice, tequila, liqueur, and salt. Let stand for 5 minutes to thicken.
Pour filling into crust. Cover with plastic wrap, and refrigerate until firmly set, for at least 2 or up to 8 hours.
Just before serving, whip cream until soft peaks form. Fold in sugar and vanilla, whipping until stiff peaks form. Garnish pie with whipped cream and lime slices, if using. Serve immediately.