Melt margarine over medium heat in a Dutch oven. Add celery and onion and cook, stirring occasionally, for 5 minutes.
Pour vegetable broth and water into the Dutch oven and bring to a boil. Add cauliflower and potato; return to a boil. Reduce heat to low and let simmer until potato is tender, about 20 minutes.
Stir red peppers and evaporated milk into the simmering mixture and heat for 2 minutes more. Pour soup carefully into an electric blender or food processor and puree until smooth. Return soup to the Dutch oven and bring to a slow boil, stirring constantly, until thoroughly heated, about 5 minutes more.