Combine the vinegar, vermouth, shallot, tarragon, and salt in a small heavy saucepan over medium heat. Bring to a boil, lower the heat, and simmer until reduced to a few spoonfuls. Cool to room temperature.
Strain the cooled mixture into another small heavy saucepan, or the top of a double boiler, and whisk in the egg yolks. Beat until thick and creamy.