Step 1 In a medium saucepan, bring 1 1/2 cups water and one-half cup sugar to boil. Add the vanilla bean and dried cherries and turn off the heat. Let it macerate until the cherries are soft and plump, about 4 minutes for lightly dried, soft cherries and 15 minutes for hard, fully dried cherries. Strain the cherries, dsicard the vanilla bean and set aside.
Step 2 In a clean medium saucepan (make sure the pan has no traces of fat in it or the caramel will crystallize), cook the remaining sugar over medium-low heat until it turns a dark amber color and the sugar is completely dissolved, about 25 to 30 minutes. Cooking time will vary depending on the size of the pan. Stir the sugar with a wooden spoon occasionally to prevent the caramel from burning.
Step 3Slowly add the vinegar 2 tablespoons at a time. Be very careful because the hot caramel will splatter and bubble. Continue to add the vinegar until it is completely incorporated, stirring occasionally with a wooden spoon. Stir in the salt and tarragon, then the reserved cherries. Simmer 5 to 10 minutes, or until syrupy. When cool, transfer to a container and keep at room temperature until ready to serve. (To clean your saucepan, fill the pan with water and bring it to a boil to melt the caramel.) Makes 1 cup of sauce.