A hollandaise sauce is an emulsion of butter droplets in water (which is the main substance in vinegar, wine, lemon juice, egg yolk, etc.), and when it breaks, the separate butter droplets have pooled together again and formed a separate layer from the water. So whisking a broken hollandaise into vinegar is essentially re-forming the emulsion, breaking the butter layer into little droplets suspended in the vinegar. It can also be a good idea to include another egg yolk with the vinegar, so that it can provide emulsifiers to coat the butter droplets and stabilize them.
Harold
