Combine vermouth, shallots, juniper berries, and peppercorns in heavy small saucepan.
Boil until liquid is reduced to 1/4 cup, about 12 minutes.
Add clam juice and boil until reduced by half, about 8 minutes.
Add cream and boil until mixture is reduced to 3/4 cup, about 8 minutes.
Add butter 1 tablespoon at a time, whisking until melted before adding more.
Strain sauce through fine-meshed strainer.
Return sauce to saucepan. (Can be made 2 hours ahead. Keep at room temperature.)
Heat oil in heavy large skillet over medium-high heat.
Sprinkle salmon with salt and pepper. Add salmon to skillet and sauté until just cooked through, about 4 minutes per side.
Meanwhile, re-warm sauce and stir in gin and lemon juice.
Season sauce to taste with salt and pepper.
Place salmon on plates and drizzle sauce over.
Garnish with chives and serve.