In a 1- to 1 1/2-quart pan over medium heat, stir shallots, 3 tablespoons white wine vinegar, and tarragon until liquid evaporates, 3 to 4 minutes. Scrape into a bowl. Let cool to room temperature, 3 to 5 minutes.
Add sour cream, Dijon mustard, and remaining 1 tablespoon white wine vinegar to tarragon-shallot mixture and mix well. Add salt to taste.
Nutritional analysis per tablespoon.