Around 1/4 of Indians are vegetarian [1]. That's 300 million people today. The vegetarian traditions of Hinduism date from a few hundred years BC. How do/did these people get B12?
Animal sources of protein. Vegetarianism does not preclude this. Most vegetarian Hindus (Brahmins mostly) are lacto-vegetarians and quite a few are ova-lacto-vegetarians.
Jains are often also pointed to as long-term Vegans in Western Vegan circles, but it turns out most Jains are a particular kind of lacto-vegetarian, with some other peculiar restrictions (no root plants). For example, most Jains will still cook with Ghee.
Some Jains live a Vegan-like lifestyle (there may be up to 500 in all of India), but wax-on and wax-off getting periodic B12 during lacto-vegetarian times (B12 is fat soluble and can stay in your system for up to 3 years). Still, B12 sufficiency is a major topic in modern Jain literature.
However, and this is important, several recent population studies in India have shown that as many as 81% of Indians are B-12 deficient. Current dietary guidelines for Vegetarians in India is driving the need for consumption of at least 4 glasses of milk per day or change to an omnivorous diet to cover the dietary B12 deficiency.
Because of cleaner food handling and the widespread introduction of antibiotics, B12 deficiency has also been demonstrated to be increasing in India and many local urban doctors report seeing multiple patients with B-12 deficiency per week.
In most Western studies, Vegetarians have far lower incidents of B12 deficiency than do Vegans. Vegan populations test as deficient at 70-90% of the population. B12 deficiency is trivially treated with animal protein sources.
[1] http://www.thehindu.com/seta/2004/10/21/stories/200410210011...