Even while following a vegan diet, it's totally possible to thick up your homemade icing if you find that it's too runny or didn't turn out as thick or luscious as you were hoping for. I recommend adding some extra powdered sugar to your frosting mixture and mix it up again. This should help thicken up your vegan cream cheese frosting. Start by adding an extra 1/4 cup of powdered sugar and see how it changes the consistency. If needed, you can add up to a 1/2 cup of extra powdered sugar to your homemade frosting. This will obviously increase the sugar in the recipe and make your frosting sweeter as well. Keep this in mind as you decide if you need to add more to thicken it up. I also suggest doing a taste test as you alter the frosting to ensure you don't end up with something too sickeningly sweet to use.