Earlier quoted context omitted.
It is easy to lack some vitamins or nutrients while going on a vegan diet. Some of them have an impact on mental health. Unless you are very comited into calculating your intake in every mineral and vitamin, I would recommend to take some supplements if you are going vegan (which is something I support but did not totally go through yet)
Look in to creatine. Studies suggest that it may increase cognition for vegetarians (but not omnivores). Again this has only been demonstrated in a couple of studies. But if you’re on a plant-based diet consider giving it a try. Omega-3 fats can also be important, but I’m not sure if there’s any vegan options there (it’s been a few years, maybe someone is synthesizing them now.)
Which Omega-3 fatty acids are you referring to? It can’t be all of them. There are plant sources like flax seeds, walnuts and others for ALA and EPA. DHA is the one that’s a bit difficult to get (or be synthesized in the body from the others). That has been solved by algal oil or algae based supplements (fish get DHA from algae).