Among omnivores, vegetarians, and vegans in the UK in the EPIC-Oxford study, the omnivores had the highest, vegetarians had intermediate, and vegans had the lowest blood B12 levels.
A systematic review on the subject found that “With few exceptions, the reviewed studies documented relatively high deficiency prevalence among vegetarians.”
Citing Seventh Day Adventist studies showing better B12 status among vegans and vegetarians than among omnivores, such as https://www.mdpi.com/2072-6643/10/6/722, shows how vegans CAN have good B12 status, but it if the goal is to demonstrate B12 status in MOST populations, this is misleading.
It is likely that among cultures such as Seventh Day Adventists in whom veganism and vegetarianism is common, B12 supplements are better recognized as essential. Among most other populations, however, veganism and vegetarianism seems to be a risk factor for B12 deficiency.