For the simple syrup: Combine the water and sugar in a small saucepan. Put over high heat and bring to a boil. Boil for two minutes and then remove from the heat. Add the dried or fresh lavender blooms. Let it steep for 2 hours.
To make the cocktail: Fill a cocktail shaker with ice. Add the vodka and 1 oz of the simple syrup. Shake well and pour into a glass filled with ice. Top with the club soda and garnish with fresh lavender.