How common is it for a Vegan or Vegetarian to be B12 deficient?

This really depends upon the study you read, and it does not seem that there is one “correct” answer. Prevalence of Vitamin B12 deficiency in vegan subjects ranged from 0% to 86.5% (variations based on consumption of vitamin B12-fortified foods, length of time being vegan, and adherence to a vegan diet). Prevalence of Vitamin B12 deficiency ranged from 0% to 81% (with variations between different subsets of vegetarians based on consumption of eggs and dairy).

*Pawlak R, Lester S, Babatunde T. The prevalence of cobalamin deficiency among vegetarians assessed by serum vitamin B12: a review of literature. Eur J Clin Nutr. 2014;68, 541–548. https://doi.org/10.1038/ejcn.2014.46.