Heat a large skillet or saucepan over medium heat and add the vinegar. When it begins to simmer, add the Brussels sprouts. Cook until the sprouts are just soft, about 5 minutes, then remove from the heat.
In a large bowl, combine the mayonnaise, buttermilk, lemon juice, sugar, mustard, celery salt, 1/4 teaspoon salt and a dash of pepper. Whisk until blended and smooth. Add the Brussels sprouts and toss to coat. Refrigerate at least 4 hours, preferably overnight.
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