In a small bowl add vinegar, salt and sugar and whisk until almost disolved (about 2 minutes).
In small food processor or blender, add vinegar mixture, mayonnaise and horseradish and pureé or blend until smooth. (You can also omit processor or blender, it will just have a few more “bits” of horseradish, but tastes delicious just the same).
Store sauce in covered container and chill for at least two hours.