In a medium saucepan, combine the blueberries, sugar, lemon juice, and water. Stir to combine and bring to a simmer over medium heat.
Once the mixture starts to simmer, reduce the heat to low and let it cook for about 10-15 minutes, stirring occasionally, until the blueberries have burst and the mixture thickens slightly.
Remove the saucepan from heat and stir in the vanilla extract. Allow the mixture to cool to room temperature.
Once cooled, strain the blueberry mixture through a fine mesh sieve into a bowl, pressing down on the solids to extract as much liquid as possible. Discard the solids.
To serve, fill a glass with ice cubes, pour in 1/4 cup of the blueberry lemon syrup, and top with sparkling water or club soda. Stir gently to combine.
Garnish with lemon slices and fresh blueberries. Enjoy immediately!