Summary by @tldr:

The lack of understanding when it comes to eating properly causes a deficiency in nutrition no matter if you are vegan, vegetarian or meat eater.
All of the nutrition you need do exist in a plant based diet, you only need to learn what to eat which in truth everyone should do in order to keep the body healthy and free from things like cancer, diabetes, heart disease etc.
It is even shown that those going on a plant based diet have less problems with nutrient deficiency than omnivores and you can read more about it in this article that is written by Dr. Michael Greger.
Furthermore, Dr Virginia Vetrano states that active Vitamin B12 coenzymes are found in bacteria in the mouth, around the teeth, in the nasopharynx, around the tonsils and in the tonsillar crypts, in the folds at the base of the tongue, and in the upper bronchial tree.
This does not involve the complex enzyme mechanism for absorption (Intrinsic Factor) in the small intestine as required by cyanocobalamin.
Here is a video that also talks about the protein combine myth that been created and debunked decades ago, it's with Dr Michael Greger from Nutritionfacts.org