The reason that people have B12 deficiencies is because their soil lacks cobalt. (Don't quote me it could be a different trace element but I'm pretty sure it's cobalt for B12)

The B12 added to meat is literally added to it. The B12 in vegetables depends on rhe soil, because nobody fortifies vegetables like they do with meat and grains.

Vegans are actually far less likely to have vitamin deficiencies because their diets tend to be more diverse.

However "junk food vegans" are obviously going to be just as if not more unhealthy than anyone else, because their diets consist of mostly potatoes and seed oils.

Veganism is afterall not a diet, it's a lifestyle choice. Its about reducing the amount of suffering and exploitation to animals.