What vegan substitutes are available for cheese?
Many vegans use nutritional yeast as a cheese substitute.

There are also "vegan cheeses" that are available.  However, check the ingredients closely as many fake cheeses contain casein and thus are not vegan.  Some vegan cheeses will melt and some will not.  I've never tried them in a sauce.

Here's a link to The Vegetarian Resource Group that has more information about vegetarian and vegan cheeses.