As I said, some people can thrive on a vegan diet, but they will need to supplement with B12 because there are no plant sources of B12.   There are some people who say that spirulina and chlorella contain B12, however, I have not seen a good source that confirms that.

There'a genetic component to how much difficulty someone will have absorbing B12 throughout their life.

Maybe the physicians recommending a vegan diet should spend a few years working with people in India who have been crippled by various neurodegenerative diseases because of B12 deficiency.   There are medical research papers published by researchers from India that carry a hint of frustration on the topic of B12 vis-a-vis treatment protocols advocated by many western doctors.