Heat the oven to 350 degrees F. Cut a piece of parchment to fit inside a large ovenproof skillet, saute pan, or flameproof baking dish. Lightly butter one side of the parchment. Combine the lemon juice, tarragon, and creme fraiche in a small bowl and set aside. Melt 2 tablespoons of the butter in the pan or baking dish over medium-high heat. Add the shallot and vermouth and reduce to a glaze; remove from the heat. Season the fillets with salt and pepper and arrange in a single layer in the pan. Pour the creme fraiche mixture over the salmon, cover with the parchment, butter side down, and bake until the salmon is just opaque throughout, 18 to 20 minutes; don't overcook.

Transfer the salmon to a plate and keep warm. Put the pan over medium heat and reduce the sauce until it's thick enough to lightly coat a spoon. Remove from the heat, whisk in the remaining 2 tablespoons butter, and taste for salt and pepper. Put each salmon fillet on a warm dinner plate and spoon the sauce over each. Garnish with the dill or chives.