I'm sure you could devise a non-meat diet for an obligate carnivore, though I'm not sure you should, and it would be complicated, and besides, cats are never good test subjects for dietary interventions. Ours are mostly turning their nose up at the kidney-care food* we're presenting them with, so good luck with a vegan diet.
*failing kidneys, of course, being a corollary of a life-long high protein diet. Take that low carb.