Grate cucumber and toss with vinegar and salt in a colander. Set aside for 30 minutes to drain. With your hands squeeze juices from cucumber. Combine cucumber in a mixing bowl with remaining sauce ingredients and season to taste with salt and pepper.
Skin salmon fillets and remove bones. Fill a wide saucepan with water, about 3 inches deep. For one quart of water, add 2 teaspoons salt and 3 tablespoons vinegar; adjust proportions accordingly for more or less water. Bring to a boil. Carefully add salmon to pan and adjust heat until water is just below simmering -- it should shiver, and not quite bubble. Cook until fish has turned from translucent to opaque and feels springy to touch, about 8 minutes.
Remove salmon with a spatula and drain on a paper towel-lined plate. Transfer to serving platter, garnish with sauce and decorate with dill sprigs.