1. In a small pot,combine 1 1/2 cups fresh blueberries,water,sugar and vanilla extract.
2. Stir well and bring to a boil over high heat.
3. Reduce heat to low and simmer until sugar is dissolved and blueberries are soft (for about 15 minutes).
4. Remove blueberry mixture from the heat and strain, pressing on the solids to extract as much liquid as possible.
5. Discard the solids.
6. Transfer the simple blueberry syrup to the glass container,cover tightly and refrigerate.
7. To 4 oz (120 ml) cocktail glass,add remaining blueberries,lime and 2-3 tablespoons symple blueberry syrup.
8. Using a muddle,smash blueberries and line.
9. Add tequila and stir well to combine.
10. Top with cubed ice and Seltzer water.