Supplements/vitamins aren't enough on the vegan diet. Your body needs a certain amount of animal fats to regenerate tissues. That's why a 30 year old vegan woman looks 40+.

But yes, long term the vegan diet does result malnourishment. At least a vegetarian can get a certain amount of animal fats from eggs or diary. Assume that any long term vegan that doesn't look older than their age and like a skeleton is not a true vegan but someone that just treats it as a suggestion.

Even on the vegetarian diet (I've been in relationships with several women that insisted on it), they start to feel depressed and have mental issues after enough time. I remember making a fish dish for one and within days she was already starting to feel in better spirits and had more energy.

I think humans can get by with a mostly vegetarian diet, but you go completely meat free for years you are malnourishing yourself. Eggs are really cheap too, just add one into a stir fry or large enough soup and you won't even notice it but your body will notice the difference if you have at least one or two a week.