This soup is already close to being vegetarian, just make sure you use veggie broth instead of chicken broth.  For a vegan cauliflower soup, replace the butter with extra virgin olive oil, and use your favorite milk-alternative instead of the heavy cream. Try a can of full-fat coconut milk.  Also add a vegan cheese, preferably one that is nut-based and has a good flavor and melting qualities. Or just leave the cheese out altogether.