Preheat your grill to medium direct heat (about 350 degrees). Grill the grapefruit until deep char marks form. Meanwhile, combine ginger and honey. Juice the grapefruit in a reserve container. Combine the honey and ginger and reserve. In two cocktail glasses, add ice, then add 2 oz. vodka per glass. Divide the honey evenly among the glasses. Add the grapefruit juice and stir. If the grapefruit juice is too tart, you can add a little water.