I am not sure about vegan diet but I have many friends who have been vegetarians(include dairy products in their food) their whole life and are healthy. Have never heard of a link between being vegetarian and vitamin deficiency. I presume as long as we eat a wide variety of vegetables and lentils, it should be fine. NHS seems to support the case as well:

https://www.nhs.uk/live-well/eat-well/vegetarian-and-vegan-d...

I believe cutting back on meat would still be a significant improvement over the current situation.

Sure, that's something to keep in mind and the reason why I did not go full vegetarian but just reduced my meat intake.

Edit: I was not talking about missing vitamins when I said "missing". I was talking about missing something as in suffering from practising self-denial.

Somehow, vegans seem to do fine. B12 supplements are necessary, and D supplementation is recommended for everyone regardless of diet where I live, but if we really could not survive without meat, I think we'd know about it by now.

B12 you get from suplements is not the same (structure) b12 you get from natural sources. There are risks of taking those if it is from cyanocobalamin.

I'm not aware of widespread, serious issues in actual vegans.

A friend of mine who is vegan fainted and only then was found to have severe B12 deficiency. Its more widespread than you think.

Currently animals raised for meat consumption get B12 injections too. What vegan do is just making the loop more efficient by taking the "injection" themselves.