Humans need amino acids that are only found in meat for our full health. This is easy enough to counteract by taking vitamin supplements
There are no aminoacids that you can’t find in plants. And there are no aminoacids that can be replaced by vitamins. Iron deficiency is a real possibility when eating a vegan diet, but B12 deficiency is a certitude for those who don’t supplement.
I was fully vegan for about 2 years. Being a clinical biologist, I’ve been having blood tests done regularly and inspecting my own blood smears. After a year or so I’ve began to see my hemoglobin and ferritin levels drop, so I started trying various iron supplements. Only one of them worked (a sucrosomial iron supplement) and started raising my hemoglobin. Sadly, I deemed it to be too slow and too expensive, so I started eating eggs again, occasionally some fish and chicken. After a few weeks I started gaining muscle mass and I saw some drastic improvent in my fitness levels (I guess I was also underestimating my necessary protein intake). After a year of eating meat and eggs, my ferritin has normalised, and so has my hemoglobin and erythrocite indices. After my experiences I’m still not going to discourage people from at least trying a vegan diet. I’ve seen enough vegans who are healthy, and I’ve seen people for whom a vegan diet is insuitable. It’s ok to quit if it affects your health.