In a 6- to 8-inch frying pan over high heat, boil garlic and shallot in vermouth until mixture is reduced by about half, 5 to 8 minutes. Add broth and bring to a boil again.
Add cream, olives, and chopped thyme. Boil, stirring occasionally, until sauce is thick enough to coat the back of a spoon and is reduced to about 1 1/4 cups, about 5 minutes. Add salt and pepper to taste. Cover and set aside.
Rinse salmon fillet and pat dry. With tweezers, pull out pin bones. Lay fillet in a buttered 12- by 17-inch baking pan. Dot the fillet with butter, drizzle with wine, and sprinkle with tarragon. Sprinkle lightly with salt and pepper.
Bake in a 400° oven until salmon is barely opaque but still moist-looking in center of thickest part (cut to test), 13 to 18 minutes. Slide salmon onto a platter.
If sauce is cool, stir over medium-high heat until hot. Drizzle fish with some of the sauce; serve remaining to add to taste. Garnish fish with thyme sprigs.
Wine pairing: A white with forward fruit and good acid in the finish, such as an Oregon Pinot Gris--we liked WillaKenzie and Adelsheim, both 2003--or a minerally French white Burgundy.