Peel cucumbers. Cut into 1/2-inch pieces; toss cucumber with salt, and transfer to a colander placed over a bowl. Cover and chill up to 8 hours.
Transfer cucumber to a food processor or blender, discarding all liquid. Puree until smooth, stopping to scrape down sides. Whisk in buttermilk, pepper, and dillweed. Pour mixture into a large bowl. Set aside.
Melt butter in a small skillet over medium heat. Add shallots, and cook 1 minute. Add shrimp, and cook 30 seconds. Add vermouth; bring to a boil, and immediately strain liquid into cucumber mixture. Place shrimp mixture in a bowl; cover and chill shrimp mixture and cucumber mixture.
Stir vinegar into cucumber mixture. Divide shrimp mixture evenly among individual soup bowls. Ladle soup over shrimp, and serve with diced tomato and razor red onions. Garnish, if desired.
*Razor red onions are red onions sliced paper thin and tossed with a pinch of sugar and enough lemon juice to coat. Prepare 1 to 2 hours ahead, and toss occasionally until softened and pink.