Place crush graham crackers on a small plate. Brush the rims of two cocktail glasses with a little simple syrup and then roll them in the crushed graham crackers to coat.
Place mint leaves and simple syrup in a shaker and muddle.
Add key lime juice, vanilla vodka, and heavy cream to the shaker Add some ice, cover, and shake vigorously.
Divide cocktail into two prepared drinking glasses. Top off with soda water and garnish with additional mint.