There are many edible wild plants that can be found in the wilderness. I'm no expert, but I know of at least forty-five edible wild plants that I can find on my farm. You would think that with that many edible wild plants available it would be fairly easy to survive on a vegetarian diet in the wilds. It is not. A life supporting diet doesn't just depend on the quantity of food available. It also depends on the quality of the food available.
The human body needs carbohydrates, proteins, and fats to survive. Wild plants have a goodly amount of carbohydrates but protein and fats are a problem. Wild fruits and vegetables have virtually no protein. Wild seeds and nuts have more protein, nearly the concentration that is found in meat, fowl, or fish. But one big drawback to getting your protein from plants is that the plants that produce protein are very seasonal. Acorns, pecans, hickory nuts, black walnuts, and pine seeds are only available in the fall. Most grass seeds are harvestable only in the fall, though some may remain of the stalk into the winter. Not much help if you are trying to survive in the spring and summer.
I think it is ironic that one of the greatest plagues of the modern diet is fat. We look for low-fat this and low-fat that at the grocery store. We trim the fat from our meat and try to avoid butter and lard. The ironic thing is that fat is one of the hardest dietary substances to find in the wild. Even wild meat is pretty low in fat. People that have to live on wild meat prize animals, like possoms, bears, or wild hogs, that have fat in the meat. Wild seeds and nuts have fat in them, but the problem is the same as stated above, they can mostly only be found in the fall.
Maybe you can enlighten me, but I know of no native culture that did or does subsist only by foraging for wild plants. All gatherers that I know of were hunter/gatherers even if they were only hunting grasshoppers. Even native groups with fairly advanced agriculture supplemented there diets by hunting. It's just hard to get enough protein and fat without eating meat.
So if you are a vegan in modern life, be warned. If you are going to survive in the wilderness, you will become an omnivore, so you better start cultivating a taste for possom right now.