There are two vitamins that I recommend that everyone -- vegan, vegetarian, or omnivore -- supplement. One is vitamin D: our bodies can make enough of it given enough sunshine, but our supplies can run dry in the dark winter, and a lot of people stay out of the sun during the summer months for skin cancer or sunburn concerns. Everyone should supplement during the winter, and if you're avoiding the sun in summer, supplement then also. (Plant-derived D2 may be slightly less efficient than vitamin D3 derived from animal fats, but so what? This is just insurance against running out of your body's natural supply, and take a little bit more if you're worried about it.)
The other is B12. Vitamin B12 is found in flesh, eggs, and dairy products but is produced by bacteria, so it is not an animal product itself. If one was really hot about not taking pills I suppose they could culture the appropriate bacteria and eat the cultures. (Or eat your own feces...our gut contains B12-making bacteria, though apparently not enough or not in the right part of the gut for it to be absorbed in most people.) But a pill is a hell off a lot less trouble. B12 deficiency is not just an issue for vegans; many cases result in omnivores from inability to absorb it adequately from food. Since deficiency is serious but doesn't show early symptoms, and supplementation is safe and cheap, buy a bottle of pills once a year or so and take one a week.
So far, of all the posts, Vegan B12 deficiency: putting it into perspective was the most commented upon. The conversation there and under the corresponding video, centered on practical questions about how someone eating vegan — no meat, dairy, or eggs — can ensure a regular, reliable source of vitamin B12. Here are the recommendations I posted:
In my professional opinion, the easiest and most inexpensive way to get one’s B12 is to take at least 2,500 mcg (µg) of cyanocobalamin once each week, ideally as a chewable, sublingual, or liquid supplement (you can’t take too much–all you get is expensive pee).
Or, if you’d rather get into the habit of taking something daily (instead of once-a-week), I recommend at least 250mcg (I know the math doesn’t “add up” but that’s due to the vagaries of the B12 receptor system — I’ll record and upload a video showing how I arrived at these recommendations).
Or, if you’d rather get it from B12-fortified foods instead of supplements, I’d suggest three servings a day, each containing at least 25% of the “Daily Value” on its label (again, I’ll explain). Such foods can be as exotic as a certain type of “nutritional yeast” or as simple as a bowl of Cheerios.