In a large saucepan over low heat, combine 8 cups whole milk, 2 ½ cups granulated sugar, and 2 tablespoons vanilla.
Dulce de leche burns easily. Stir steadily with a spatula; the mixture will slowly turn caramel brown and the bottom of the pan will be visible.
Stir constantly for at least three hours, until the caramel has thickened to a puddinglike consistency. Remove from heat. Cool 10 minutes; strain through a sieve. Refrigerate for up to one month.