For the potato salad, cover the potatoes with cold water in a medium saucepan; add 1/2 teaspoon salt. Heat to a boil over high heat; cook until just tender, about 15 minutes. Drain well; transfer to a large bowl filled with ice and water. Set aside to cool, 30 minutes. Peel, if desired.
Combine the vinegar, remaining 1/2 teaspoon of salt, sugar and black pepper to taste in a medium bowl. Cut each potato crosswise into discs about 1/4 inch thick. Add potatoes and red onion to the vinegar mixture; toss gently. Cover; refrigerate 3 hours.
For the cucumber-yogurt sauce, combine the cucumbers, yogurt, creme fraiche, dill, lemon juice and zest and onion in a medium bowl. Stir gently to combine; season with the salt and black pepper to taste. Cover; refrigerate.
For the salmon, prepare a grill for medium-high heat. Brush the salmon fillets on both sides with the oil; season with the salt and black pepper to taste. Grill, turning once, until cooked through but still moist, 8-10 minutes.
Meanwhile, place the potato salad in a large skillet; add the chicken broth and butter. Cook over medium heat, stirring occasionally, until heated through, about 4 minutes. Taste; add more salt and pepper, if desired.
Spoon the potato salad onto 4 plates; place a fillet over each bed of salad. Spoon some of the cucumber sauce over each fillet and around the plate. Garnish each serving with a dill sprig. Serve remaining sauce in a bowl at the table.