Hi there, and welcome!
The topic of vegan cats has been discussed several times on these boards. If you do a search you might be able to find some threads on it. Basically, there are two schools of thought about feeding cats a vegan diet. One is that if you feed them meat-based diets you are contributing to the cruelty of killing animals and the diet is not even really what cats would eat in the wild (rats, mice, insects, etc.). The other is that a cat is a natural carnivore and it's cruel to try to make him eat a diet that goes against his biology.
That said, some people have success at feeding cats a vegan diet.
http://www.vegancats.com talks about this more. Personally I was quite adamant about feeding my 2 cats a vegan diet, but despite emailing the man in charge of vegancats.com and trying lots of his suggestions including enzyme supplementation, one of my 2 cats was truly not thriving on a vegan diet. At first she seemed fine, and for 6 months or so didn't seem to have any problems. But after a while she'd lost a lot of weight, her coat was not shiny and she looked like she couldn't even see right, there was blood in her stools and she vomited a lot. I just looked at her one day and thought, I can't stand to watch her deteroriate anymore. I'm a vegan because I love animals, and I can't watch this animal suffer like this. So I bought her some organic meat-based catfood, and she started improving right away. My other cat seemed fine on a vegan diet. So I really think it depends on the cat. But if your cats don't thrive on a vegan diet, you have remind yourself that it's just not a natural diet for them, any more than a carnivorous diet would be natural for us.
Personally, I really never want to have any more cats after these ones pass on, because it's too hard of an ethical dilemna for me to deal with, and you never know if your cat will or will not adjust well to a vegan diet. I'd rather not chance it, unless a cat was in dire need of rescuing (as these two cats I have were when I took them in).