Combine the granulated sugar, brown sugar, and water in a small heavy saucepan. Cook over medium-high heat until the mixture turns dark amber, washing away any sugar crystals clinging to the inside of the pan with a damp pastry brush.
While the sugar is caramelizing, heat the milk to a simmer in another saucepan or in the microwave. When the sugar is fully caramelized, stir the warm milk into it. The sugar will immediately crystallize, and the milk will vigorously bubble and steam. Stand back so you don’t get burned. Then, as the bubbling subsides, stir the caramel until it becomes smooth and fluid again. Remove from the heat, stir in the vanilla, and let cool for 5 minutes.
This syrup will keep in the refrigerator for up to 2 days, but must be warmed until liquid before using to make a soda.
Divide the liquid caramel among four tall glasses. Stir 1 cup seltzer into each. Fill the glasses with ice and serve. To mix with seltzer 1 batch caramel syrup 4 cups seltzer.