In a large bowl, whisk to combine sweetened condensed milk and vanilla. Add lemon juice and lemon zest and whisk until thickened. If desired, stir in a few drops of yellow food coloring to give the pie a brighter yellow color.
Pour lemon filling into graham cracker crust.
Cover and freeze or refrigerator overnight, or for at least 4 hours.
Slice and serve topped with a dollop of whipped cream and/or slice of lemon, if desired. If serving from frozen I suggest letting the pie sit at room temperature 15-20 minutes to let it soften up just a little bit.
Notes
Store covered in the refrigerator for up to 5 days. Or for longer storage freeze for up to 3 months. Just be sure to wrap tightly in plastic wrap before freezing.