@Tiki
I found an interesting article that may explain how traditional vegetarian Hindus may have avoided B12 deficiencies.

http://www.vegsource.com/articles/walsh_byrnes_b12_print.htm

From what I understand, B12 is a bacterial byproduct, and not specifically associated with either meat or vegetable products. In essence, wherever B12 producing bacteria exist, you can get B12.

The B12 we get from our animals like cows, for eg, is from B12 producing bacteria that exists in their stomachs from plants that they have consumed. We then ingest the B12 produced by these bacteria when we later consume the animal.

Some have theorized that originally, Hindus consumed plant material with B12 bacteria deposited on it from other sources such as insects , and because they did not follow the regimented sanitation procedures we use, B12 bacteria survived and continued to produce B12.

It would appear that modern day vegans have to supplement their diet with B12 and others because we are much more careful to destroy all bacteria during the processing of our food.

There is also evidence that the incidence of serious B12 deficiencies in contemporary immigrant vegetarian Hindu families in the UK, for instance, rises once they get there from India.

Presumably because the natural B12 bacteria laden foods they normally got while in India are unavailable due to the rigorous cleaning standards followed in the UK. They are usually forced to consume fortified foods or take supplements in order to avoid B12 and other deficiencies.