Well, it’s just not Easter dinner without dandelion salad, but sometimes when Easter comes early, and you’re in a cold climate, there just isn’t any on the ground yet. And while it’s fun and not too time consuming to harvest it, cleaning it takes way too much work. So yes, a few times, I’ve cheated and just bought it from the store.
And the plants are around for most of the year, but they get very bitter after they bloom. That’s what limits their season.
Anyone wants to come to my lawn all summer can have all the dandelions any person could want.
Seriously, the lack of B12 will doom any vegan diet. Does anyone know if the supplements are from an animal source. Incidentally, I think ants are a good source.
Although it may be that dandelions and corn are good sodium sources, mightn’t it depend on the amount of sodium in the soil? Also I wonder if some other trace nutrients might be missing in such a restricted diet.
Come to think of it, would that diet have enough fat? That’s another essential nutrient that Americans don’t usually need to worry about, but which is relevant for extreme diets.
Corn and soybeans have a good amount of fat, hence corn and soybean oil.
Herbivorous animals get much of the B12 they need from insects and other small animals that they ingest with their food. So you might be OK if you let weevils develop in your rice and beans and didn’t remove them when you cooked them. 
So where do the bugs get the B12 from? At some point there has to be some organism that actually produces the stuff.
(and why do I get the feeling that the answer is going to be “symbiotic gut bacteria”?)
It appears to be that way. Even the B12 humans produce in their guts can’t be absorbed, although it leaves open the possibility of humans being the source of their own B12.
Marcus Flavius, are you ruling out consuming animal feces?
No need, B12 supplements exist.