From the 1970s through the 1990s, there was a widespread belief that vegetarians and vegans ran great risks of developing a protein deficiency. Much of this concern is traceable to the seminal pro-vegetarian book Diet for a Small Planet, which in hindsight offered protein recommendations that were needlessly demanding.
That said, it’s indeed possible for a vegan to take in insufficient protein, but to do so in the absence of some sort of metabolic abnormality likely requires some combination of eating too few calories and avoiding rich sources of protein. Fortunately, there are a number of great vegan sources of protein, so it’s simple to eat in a way that more than covers your protein requirements.
Rich Vegan Protein Sources:
For thorough coverage of how to ensure your protein needs are satisfied, see Jack Norris RD’s excellent protein article at VeganHealth.org