Probably this soup should be called Potato and Leek Soup with Jerusalem Artichoke, because once you add the Jerusalem artichokes, it is not, technically, a Vichysoisse.
Jerusalem artichokes are winter vegetables, so probably lend themselves to hot soups rather than cold, so it is not really a Vichysoisse anyway, if you follow my reasoning. Whatever you want to call it the variation is a delicious one. Jerusalem artichokes are not a bit like the globe artichokes that Kate would almost kill for. They look like a cross between fresh ginger and a very knobbly potato and have a stunning flavour like nothing else. When you add it, someone always asks, “What is the wonderful flavour in this soup?”
To make, simply add 3-4 (or more, if you find the flavour addictive) Jerusalem artichokes to the Vichysoisse recipe above. Peel them and treat them exactly as you would the potatoes.