In a small saucepan, combine vinegar, vermouth, shallot, thyme, salt, and pepper over medium-high heat. Bring to a boil, and cook for 5 to 6 minutes, stirring occasionally, until mixture is reduced to 2 tablespoons. Cool for 5 minutes. Add egg yolks, whisking until smooth.

Put mixture in the top of a double boiler over simmering water. Cook for 1 to 2 minutes, whisking constantly, or until mixture begins to thicken. Remove from heat, and gradually whisk in butter until melted. Whisk in lemon juice. Serve immediately.