I'd have to spend more time than I have hunting the relevant studies down. I don't even pretend to understand most of the fundamental science, but my very cursory studies show that long-term vegan diets are pretty much a terrible idea for humans, despite what vegan advocacy literature proclaims, there are no populations of humans on the planet that go for a lifetime (or generations) on a purely vegan diet. Most of the ones used as examples, have some animal sources in their diet but those sources are not recorded or well understood by outside observers who are also biased by trying to advocate for a particularly philosophical dietary proscription.

Just with respect to the B group vitamins, there's quite a few studies of vegan populations that show a consistent deficit in B12. Depending on the study, vegan populations consistently show (at 70-90% of the studied population) B12 deficits, even while taking supplements. The general conclusion among nutritionists is that it's very very hard for vegans to get enough B12 in their diets no matter what they try. Strangely, these studies never seem to show up in Vegan advocacy circles or support groups, though there is a general awareness that B12 availability is a problem with the diet. More problematic, B12 deficiency (and the related issues below) do not present as immediate health issues, but develop over years and decades - presenting as permanent damage to neurological tissues. People who switch to Vegan diets often don't present with this deficiency for several years as the liver maintains a 3-5 year store of the vitamin.

B12 deficiency can be fatal. B12 deficiency is trivially treated by eating meat - surprisingly little meat too. Similar studies of Vegetarian populations who occasionally eat some animal source, don't show nearly as frequent B12 deficiencies as in Vegan populations.

Identifying B12 deficiency can be difficult as normal assays don't reliably show deficiencies, and initial symptoms looks like anemia, which is also highly prevalent among vegan dieters and may co-present.

Along with B12 deficiency, elevated methylmalonic acid is typical in some majority of the studied vegan populations. Which is used as a secondary signal to confirm B12 deficiency. MMA reacts with B12 to produce coenzyme A (CoA) which is necessary for normal cell function. People with elevated MMA are often diagnosed with methylmalonic acidemia - a group of disorders which are normally only seen in infants with specific genetic conditions relevant to B12 absorption, but present with progressive encephalopathy and hyperammonemia, both can cause death over the long-term.

A majority of vegans can also usually be diagnosed with Hyperhomocysteinemia (which is only ever seen in three population groups, the severely malnourished, the extreme elderly and vegans and is trivially treated with an omnivorous diet). Studies which show cardiovascular issues in Vegans (surprisingly more common than you'd think) attribute them to Hyperhomocysteinemia as Homocysteine prevents proper formation and maintenance of key cardiovascular components like collagen and elastin (also noticeable in Vegan populations as "thin skin" or "premature wrinkling"). It's also linked to Alzheimer's disease and Schizophrenia. The later disease occurs in vegan populations with a startlingly higher frequency than in vegetarian and omnivorous dieter populations.

This is just some of the issues from B12 deficiency, which occurs somewhere between 70-90% of any randomly selected vegan population. So we know that vegans, as a general population, are not able to supply themselves with sufficient B12.

You can probably google for the relevant studies on your own. There's plenty of them around, but I'd urge you to find them in medical journals, not interpreted through vegan advocacy sites which seem to have a habit of ignoring the important health issues and suppressing negative health outcomes from the diet.

I'd urge you to check out amino acids, and fatty acid issues as well, as they are just as common in vegan populations and can be just as health concerning.

Interestingly, almost every single health issue that stems from a vegan diet is trivially treated by just eating a few grams of animal protein sources a week. Eating plant foods or supplements rich in these normally vegan deficient nutrients doesn't mean they're bioavailable, what matters is absorption of the relevant nutrient, which is something that vegan diets are notoriously poor in providing.

Here's something I wrote on this a while ago. https://news.ycombinator.com/item?id=6888323