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. Rates for Vegans and B12 deficiency range from 44-92% (variations based upon length of being a vegan and consumption of B12 fortified cereals and/or supplement use) and for vegetarians from 0-86.5% (with significant differences between the different subsets of vegetarians and whether or not they use supplements).