The B12 argument is used as a tactic to convince people that meat and dairy are necessary.  
Humans historically got B12 from eating vegetables, fruits and drinking fresh water from springs and streams, or by farming and then eating without washing their hands. Since we no longer do these things, “natural” plant based sources of Vitamin B-12 have dropped out of modern life.
In the modern day, our produce is heavily sanitized and chemically cleaned to meet retail standards, and subsequently the B12 (bacteria) removed.
The "natural" way of consuming B12 - from unwashed vegetables and unfiltered water - isn't safe for humans because there are also other less desirable bacterias present in these places such as E. coli and salmonella. So in the modern world it's much safer to just get our B12 from a supplement.
So what is B12 and why is it important?
Wikepedia:  "B12 is a water-soluble vitamin that has a key role in the normal functioning of the brain and nervous system via the synthesis of myelin, and the formation of red blood cells. It is one of eight B vitamins."
Even meat is fortified with B12 these days, as it is unnaturally raised and animals become deficient. That said, cows and pigs tend to eat their own poop (accidentally mostly,) so they tend to always retain some B12.


So, go vegan and take a B12 supplement as needed.