In a saucepan combine the shallots and champagne. Scrape the vanilla seeds from the split pod into the pan and add the vanilla pod halves. Bring to a boil and cook until almost all liquid evaporates, about 6 minutes. Add the heavy cream, bring to a boil and simmer until cream thickens and coats the back of a spoon, and is reduced by nearly 50% in volume, about 3 minutes. Whisk in the butter until smooth and strain the sauce through a fine mesh strainer, pressing on the solids to extract as much liquid as possible. Season to taste with salt, pepper and vinegar and serve immediately over fish or shellfish.
Note: If sauce is made in advance, keep warm in a bain marie over barely simmering water.
Yield: about 1 generous cup, 4 servings