HEAT 1/4 cup of the dressing in large stockpot on medium heat. Add leeks; cook 5 min. or until tender, stirring occasionally. Add potatoes, water, salt and pepper; cover. Bring to boil. Reduce heat to medium-low; simmer 15 to 20 min. or until potatoes are tender. Cool 10 min.
ADD leek mixture, in batches, to blender; cover. Blend until pureed. Return to stockpot.
STIR in milk and remaining 1/4 cup dressing; cook on medium heat until heated through, stirring frequently. Sprinkle with chives just before serving.