Melt butter in a large pot over medium-high heat and add leeks and carrots. Cook for 12-15 minutes, stirring occasionally, until leeks are wilted and carrots are softened, but make sure not to brown the leeks. Stir in Vadouvan and cook 1 minute more, then add the water. Bring mixture to a simmer, then reduce heat to medium-low and simmer 30-40 minutes, or until the carrots are tender. If using a blender to pureé the soup, let soup cool to room temperature before blending (to prevent hot soup from erupting from your blender). If you are using an immersion blender to pureé the soup, you can blend it while the soup is hot. Season to taste with salt and add more water if you would like a thinner consistency. Soup can be served hot or chilled.