Nothing is missing from a vegan diet except B12. Calcium can be found in dark green veggies. Soymilk for example, because of its typical fortification, has more calcium in it than cow milk, and without all the crap. Protein is also never a problem. It also exists in plants, as well as grains (bread products), legums, seeds, and nuts. Plant protein is far better for you than meat protein because it does not contain fat, cholesterol, drugs that were pumped into the animal, and anything disgusting the animal ate. Iron can also be located in dark greens and raisins.
Every other vitamin can be located in one plant or another, or even from sunlight (like D). It's total BS that vegan diets cannot sustain a person, short of B12 which can come from fortification. Almost every cereal out there today is currently fortified with B12. Many vegan "products" also have lots of B12 fortification. There is also some debate that ocean veggies (Kale) may contain B12. In other words, B12 can be had.
If someone isn't getting the nutrition they need, it's simply because they are not eating what needs to be eaten. Living on crackers and Oreo cookies is also vegan, but that's not going to sustain you. This also applies to meat eaters, because meat alone will not sustain you. Plant-based nutrition is the largest factor of survival for a human. Meat does not provide nearly enough of what a human needs, which is why very few people (if any) are strictly carnivores, yet many can and do survive strictly as herbivores.