Zest one lemon finely, then squeeze enough lemons to measure the juice.
Whisk lemon juice, sugar, zest, and salt in a large pitcher until the sugar dissolves.
Whisk in the sweetened condensed milk until the mixture looks smooth and creamy.
Stir in the cold water, then add the vanilla extract if using.
Fill glasses with ice and pour the creamy lemonade over the ice. Stir gently and serve.