Here are two bakeries in the St. Louis, MO area that could bake the vegan cake for you:
http://www.blackbearbakery.org/
http://sweetartstl.com/
They both offer vegan cakes, so you could place an order with either of them.
Another thing you could do is make your own cake! This cake looks like fun; if I lived in the area, I would volunteer to make you cake for you, if you wanted me to. It would be a blast to make this cake!
I saw a picture of a Kung Fu Panda themed cake on google: http://www.coolest-birthday-cakes.com/images/coolest-kung-fu-panda-bear-cake-9-21106567.jpg
Basically, you will need to bake two cakes in circle pans. Here is what the baker said about how she created her cake: "I used a 8 inch pan for head and 9 inch for the body. You will need to carve a little piece out of 9 inch cake so that the head fits just right. I used a chocolate cake mix and whipped vanilla frosting. The ears, paws and eyes are made with Oreo cookies. While cake is cooling off prepare decorations. You will need enough Oreos for ears and etc. (about 6) just twist and scrap off filling. I also crushed Oreos for arms and used crushed chocolate chip cookies (about 5) for pants.
Graham crackers would work for pants as well, just didn't have any. Finished off with red and yellow icing for belt and blue for Po's eyes."
To veganize her recipe, I'd suggest you make the following (I will link the recipes in the sources section):
Vegan chocolate cake
I would suggest a vegan cream cheese frosting, but you could also make a vegan vanilla buttercream frosting, or a vegan whipped topping, such as Rich's or Soyatoo.
Oreos are vegan, but for a healthier choice, go with Newman O's (available at Whole Foods).
As for the pants, you could use crushed Nabisco Grahams (Original), or crushed Anna's thins, in either Almond Cinnamon or Ginger flavors. I usually find Anna's thins at Big Lots...I never see them anywhere else; but that is just my case.
For the red and yellow belt, and the blue in the panda's eyes, just add food coloring (or natural food coloring, such as beets, strawberries, blueberries, and a tiny pinch of tumeric, which will not affect the flavor) to your frosting. I think you could also use jelly beans for the belt. =)
Good luck with your son's birthday, whether you choose to order from the bakery or bake at home! =)