Combine graham cracker crumbs, melted butter, and sugar in a bowl. Mix until well blended. If it seems too dry, add 1 more tablepsoon of butter.
Whisk together sweetened condensed milk, lemon juice, salt, lemon zest, and vanilla extract until smooth.
Gently fold whipped cream into the lemon mixture in two or three additions. Pour into the cooled crust and refrigerate for at least 2 hours or until set.