Preheat the oven to 375°. Arrange the fish on a parchment-lined baking sheet, spacing evenly. In a small bowl, mix the butter, capers and dill. Season the fish with salt and pepper and top with the butter mixture. Bake until the fish is opaque in the center, 12 to 15 minutes.
Meanwhile, in a large bowl, whisk the EVOO, lemon juice, vinegar, shallot, garlic and Dijon mustard; season the dressing.
Prepare a large bowl of ice water. In a large skillet, bring a couple of inches of water to a boil. Salt the water, add the asparagus and cook until crisp-tender, 2 to 3 minutes. Using a slotted spoon, transfer to the ice water and let cool. Drain and pat dry. Cut on an angle into 2-inch pieces and add to the dressing. Bring the asparagus water back to a boil and cook the peas until crisp-tender, 2 to 3 minutes. Drain in a strainer and run under cold water until cool. Drain and pat dry. Add the peas and tomatoes to the dressing; toss and season.
Divide the salmon among plates; top with the vegetables. Garnish with the tarragon.