Snap the sprig of rosemary in half. Set half aside to use a garnish, and place the other half in a cocktail shaker with a handful of ice. Pour the vodka, simple syrup, and cranberry juice over. Put the lid on the shaker and shake to combine the ingredients.

Pour the vodka and juice mixture through a strainer and into a glass of ice. Add the ginger beer and add the half sprig of rosemary to garnish.

If you are serving this at a party, you could combine the vodka, simple syrup, cranberry juice, and rosemary in advance and just add the ginger beer before serving.