Veganism is almost viable. Almost. https://en.wikipedia.org/wiki/Vitamin_B12 Human metabolism cannot make cobalamin, nor any of its analogues. The only known non-animal source is seaweed.
But the meat you eat is fed B12 to give it the B12. Without supplementation they're chronically deficient as well. Annuals are not the best way to get your B12.
I previously suffered from chronic B12 deficiency on an omni diet and was supplementing B12 with a megadose 2x every week. Doing a supplement 1x a week with a vegan diet, my B12 blood levels are better than ever.
If animals can be given it, shouldn't there be some way to introduce it into plants as well?
We used to get it from traces of ruminant animal waste on plants that weren't washed thoroughly. That's how plants had it. If you mean genetic engineering, why bother. Plants lack a digestive tract to grow the bacteria themselves. That and there's already readily available supplements made by bacteria. ---