Juice the grapefruit or measure out the 6 ounces of juice. Strain it to get rid of the pulp and pour it into a cocktail shaker with the vodka. shake it for 2 min, or until mixed. Pour the water onto a small plate and get the rim of the martini glass wet. Put the brown sugar on another plate and roll the rim of the glass in it to coat it evenly. Pour in the grapefruit and Vodka mixture and mix it together with the vanilla.