As vegans we must supplement B12 as there’s no natural vegan source (even omnivores are getting B12 as a supplement, since the animals they eat are given B12 supplements instead). A B12 deficiency can lead to physical and psychological symptoms, including irreversible nerve damage, fatigue and difficulty thinking. If you’ve only become vegan recently, it’s quite possible that you still have some B12 stores from when you ate animal products, but this will be almost completely depleted within 3 years.
The majority of people living in developed countries are not getting enough Vitamin D. You generally need to spend at least 20 minutes in direct midday sunlight to get enough, but for most of us with indoor jobs, that’s simply not possible. A vitamin D deficiency has been associated with poor bone growth, cardiovascular disease and cognitive impairment.
Finally, the majority of people, vegan or not, don’t get enough Omega-3, specifically the fatty acids EPA and DHA. It’s possible to get enough by eating multiple servings of chia seeds and flax seeds, which produce the fatty acid ALA, but the conversion to EPA and DHA is very inefficient, which is why leading doctors like Dr. Michael Greger, recommend supplementing with an algae-derived Omega-3. Omega-3 has been shown to help protect our brain as we age. Here’s a great video on it.