When you say vegan, i presume you mean they will be raised on a veg diet and no pellets etc. In the nicest posible way, I dont think you should pursue this idea because its not natural for the birds. They will have to be cooped up 24/7 and this is completely against the welfare standards set out, unless they have a very large pen.
My other idea of vegan chicken nuggets is the ones you can buy and probably did buy. They are made from chickpeas and artichoke hearts and coated in bread crumbs, and sold as vegan chicken nuggets.
By all means you can grow chickpeas and artichokes and make your own, but if the first idea is what you are pursuing I strongly advise against it.