Combine the vinegar, wine, shallots, and tarragon in a small saucepan. Boil over high heat until the mixture is reduced by half, about 1 minute. Remove from the heat, and set aside.
Melt the butter in another small saucepan over medium heat.
Combine the egg yolks and reserved shallot mixture in the top of a double boiler and place over hot, not boilin