Make the simple syrup -

Combine the blueberries, water, sugar, and vanilla extract in a small pot. Bring to a boil over high heat. Lower the heat and simmer until the sugar is completely dissolved and the blueberries have burst and softened, about 15 minutes. Remove from heat and strain the mixture, pressing on the solids to extract as much liquid as possible. Discard the solids or save them for another use, such as an ice cream topping. Transfer the simple syrup to a glass container with a tight fitting lid. Simple syrup will keep for a couple of weeks in the refrigerator. To extend the life of the simple syrup, add about a tablespoon of vodka. Your syrup will last MONTHS this way. Makes about 1 1/2 cups of simple syrup.

Make the smash -

To a 4 oz glass, add blueberries, lime, and simple syrup. With a muddler, or the handle of a wooden spoon, smash the blueberries and lime until you released their juice and the limes are fragrant. Add the tequila and stir quickly to combine. Add ice and top with seltzer.