What is vegan and what is not vegan is going to differ depending on what each vegan believes. Diet is like being religion, you going to have different people drawing the line at different points.
If you avoid meat because you do not want the nutrients or is allergic, it is a diet. If you avoid meat because you find it wrong, it is a philosophy or religion.
If the question is about belief, I would not consider "vegan" a diet in any possible meaning of the word.
(The above does not state that I find veganism to be wrong—rather, it merely classifies the ideology.)
If the question is about belief, I would not consider "vegan" a diet in any possible meaning of the word.
(The above does not state that I find veganism to be wrong—rather, it merely classifies the ideology.)