AIs Wilton's cake decorator icing vegan?

No it isn't. The sugar in it isn't vegan. 

Couldn't you just make your own decorator frosting and make it vegan? Vegetable shortening is vegan. You just have to find a powdered sugar that is. 

Look at the chart halfway down the page to find sugar companies that are vegan. http://www.vrg.org/journal/vj2007issue4/2007_issue4_sugar.php

If you are making this for a special order, be sure to charge for that sugar as you may have to order it!