In a small saucepan combine lemon juice, zest, water, granulated sugar, and vanilla extract. Over medium/high heat, bring to a boil. Reduce the heat to low and allow the mixture to simmer until slightly reduced and sugar is completely dissolved, about 15 minutes. Turn off the heat and allow the syrup to come to room temperature. Strain the syrup into a glass container with a tight fitting lid and store in the refrigerator until ready to use.

Make the lemon soda:

To an 8 ounce glass, add the simple syrup. Add ice and top with seltzer. Give it a quick stir to combine and garnish with a slices of lemon, if desired.