Whisk lemon juice, sugar, and salt in a large pitcher until the sugar dissolves.
Stream in the sweetened condensed milk while whisking until smooth and creamy.
Pour in the cold water and stir to fully combine.
Stir in vanilla. Taste and adjust sweetness or tartness to your liking.
Fill glasses with ice and pour over the creamy lemonade. Serve right away or chill 10 minutes.