Peel the cucumbers. Using a small spoon, or a melon baller, scrape the seeds out. Cut into 1/2-inch pieces; toss the cucumbers with the salt and transfer to a colander set over a bowl. Refrigerate for 2 to 3 hours or overnight.
Transfer the cucumbers to a blender or food processor, discarding all exuded juices. Puree until smooth; pass through a food mill if desired. Whisk in the buttermilk and dill; set aside.
Melt the butter in a saute pan over medium heat. When the butter foams, add the shallots and cook 30 seconds. Add the shrimp and stir over medium heat, 10 to 15 seconds. Add the vermouth, bring to a boil, and immediately strain into the reserved cucumber soup. Spread the shrimp and shallot mixture on a plate to cool, then refrigerate. Chill the soup.
To serve: Adjust the consistency of the soup with more buttermilk if necessary. Season with vinegar, salt (add sparingly; it probably won't need much), and pepper to taste. Divide the shrimp between the chilled soup plates. Ladle the soup over the shrimp and garnish with red onion, chopped tomatoes, and bits of dill strewn about.
Razor red onions are red onions sliced paper-thin and then tossed with a pinch of sugar and enough lemon juice to coat them. Prepare 1 to 2 hours ahead and toss occasionally, until softened and pink.
Copyright Ben and Karen Barker's Not Afraid of Flavor (University of North Carolina Press, 2001)