I researched vegan cat food when I first became a vegan. And I am just not comfortable putting my cat on a vegan diet. There is no way I could 100% monitor everything. And I believe that cats are obligate carnivores. Maybe you could suggest a food that only uses by products? For example, there is a pet food company that uses parts of an animal that was slaughtered for human consumption, but the cuts were too small to be packaged and sold, or it came from a part that wasn't going to sell. The mean would have otherwise gone to waste. It's not perfect but it is something.
That sounds interesting, could you post which company that is? Thanks!