Combine in a medium saucepan, sugar, vermouth, lemon and lime zests, and 2 3/4 cups water. Bring to a boil, stirring occasionally. Remove from heat. Set aside to allow lemon and lime zests to steep in mixture, about 10 minutes.
Strain, discarding zests. Pour mixture into a 13 by-9-inch glass baking dish. Freeze for 8 hours or overnight.
Using a sharp knife, cut off tops and bottoms of grapefruits. Remove peel, pith, and outer membranes. Working over a bowl, cut fruit segments from membranes. Squeeze juice from membranes from into bowl; set aside.
Just before serving, remove granita mixture from freezer. Using the tines of a fork, scrape surface of granita to resemble crushed ice. Transfer granita to serving bowls and garnish with grapefruit sections and reserved juice.
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