A truly vegan society would eventually go extinct if they didn't supplement with B12. To my knowledge, this isn't possible without modern technology.

It can take many years for freshly minted vegans to have their stores of essential nutrients depleted, so they may in fact reproduce successfully for one generation, but in the long run severe malnutrition is inevitable.

Okay = survive long enough to leave offsprung, and be strong enough not to be murdered by your neighbours.

Of course there is more to healthiness thsn that and science is important, but as far the rule of thumb goes, the second part is important.