Beat sweetened condensed milk, egg yolks, lemon juice, and lemon zest together in a bowl until smooth; pour into pie crust.
Beat egg whites with an electric mixer in a glass or metal bowl until foamy. Gradually add sugar and vanilla extract, continuing to beat until stiff peaks form. Lift your beater or whisk straight up: the egg whites will form sharp peaks; spoon onto the pie to cover completely.
Bake pie in preheated oven until the tips of the topping begin to brown, about 3 minutes.
