sugar
2 ounces vanilla vodka
1 ounce fluid ounce limoncello
ice cube
1 dash carbonated lemon-lime beverage (recommended Sprite or 7-Up)

Moisten the rim of a martini glass and coat with sugar. Add vanilla vodka and limoncello to a martini shaker filled with ice. Shake and then strain into the martini glass. Add a splash of lemon-lime soda.