Melt margarine in a large stockpot over medium heat; cook and stir onions, celery, and carrots until tender, about 10 minutes. Add water, vegetable bouillon, salt, and pepper; stir well. Add potatoes and bring soup to a boil; reduce heat and simmer until potatoes are tender, about 15 minutes.
Mix 4 cups soy milk and instant potatoes into soup until dissolved.
Stir remaining 1/2 cup soy milk and cornstarch together in a bowl until smooth; add to the soup, stirring well. Cook soup until heated through, stirring regularly, 5 to 10 minutes.