I don't need to cite anything, 10 seconds on google scholar looking for long term studies will suffice for your needs.

Good luck with it and be careful.

tl;dr

Here's your 100 day challenge, get 100% of your dietary requirements from your diet (without fortification of key ingredients or supplements, just from pure foodstuffs), and prove to me that you aren't eating a poor diet.

here's the tl part:

You're overfocusing on B12, because the literature on vegan nutrition overfocuses on B12 as neurological damage caused by B12 deficiencies are generally irreversible. Dietary B12 doesn't come from many non-animal sources, but you don't need much anyway, and effects of B12 deficiencies aren't symptomatic for several years. Your pre-vegan intake of B12 would have been sufficient for a few years until you figured out non-animal sources of it. It's also unclear to date if the form of B12 found in non-animal sources (synthetic eukaryotic sources, etc.) functions identically to animal sourced (from prokaryotic sources) B12. There are almost no studies on it because you have to try and be deficient in B12 on a normal diet without large-scale intestinal disorders.

"Even B12 is only accumulated in animal tissue, it's not produced by animals."

I don't think you understand where B12 comes from, I'm sure your understanding came from some vegan promotional literature which seems woefully full of cherry picked misunderstandings and absurd apologetics.

I'm sure you're getting it through some supplement. Now get it through dietary means. Or if you actually care about the environment, get it from locally produced dietary sources alone and don't have to have some heavily processed fermented foodstuff shipped a thousand miles to your fridge. I don't need to cite anything for you to know you simply can't.

α-linolenic acid is where you'll probably have the biggest long-term health issues, as metabolism into EPA and DHA is very inefficient with most of the issues in DHA production. Multiple studies show deficiencies in vegan diets w/r to DHA. Synthesis, requires several dietary co-factors in careful balance and long term studies suggest possible liver damage in humans (but not conclusively) vs. simply ingesting animal sources of long-chain n–3 fatty acids. Most vegans eat sources of ALA thinking it will makeup for their dietary deficiencies EPA and DHA, but multiple studies show that DHA levels remain deficient in these cases.

I'm sure you take supplements to make up for this inadequacy in your diet. Now do it without them. And if you think that your non-animal sourced Omega-3 supplement is complete, think again. There is no such thing as one kind of ω−3. You need them all, but in particular you need EPA and DHA.

Oh and DHA supplements also have a nasty side effect of preventing blood clotting, damaging immune response and increasing LDL levels. In other words, don't take them, they will hurt you.

Vitamin D should also be a no-supplement required vitamin. You make it in your skin for goodness sakes! It's not really essential, but it's plentiful in animal sources, and just stepping out in the sun for a bit everyday is more than sufficient to produce all the D secosteroid you could ever possibly need. If you feel the need to take D supplements, for all that is holy, take D3 and not D2 as the bio-availability of D3 is several times higher than D2. But of course, if you're taking "vegan-friendly" D supplements, it's almost always a fungal source which of course is the deficient D2 ergocalciferol form. I would suggest UV lamps at your desk instead of supplements.

I'm sure you already know about Iron and Zinc deficiencies in your diet. Every vegan I know is acutely aware of it and tries to eat lots of iron and zinc rich food stuffs and takes yet again more supplements to make up for their shitty diet.

But look, the point is this: the definition of a poor diet is a diet that doesn't provide for all of the necessary nutrients as part of the diet. If you have to take supplements to make up for dietary shortfalls, your diet is a poor one...period. Waiving away the handful of supplements you take everyday is madness and a serious problem.

Vegans are among the only otherwise healthy population group in the developed world that routinely suffers from illnesses seen only in the most decrepit poverty stricken parts of the undeveloped world. Most vegans source their information from highly biased vegan promotional material and don't understand the basic science.

I get it, you want to help the animals out of some sort of moral obligation. And I'm sure you only eat food produced on farms with no field kills, and use the parts of the internet only on a machine powered by sources that have no animal impact. And that somehow you live in a vegan mecca where somehow all of the various plants that produce 100% of your dietary requirements don't have to be shipped from halfway around the planet killing goodness knows how many animals in the process and that you believe in a world where everybody else goes vegan and the billions of domesticated farm animals somehow continue to find sponsorship for their care and maintenance in perpetuity. I think that's great.

Just be careful, and proceed with the understanding that long-term, you won't be able to sustain this diet without compromises to your health.