Protein powder as main protein source? Yes or know and why?

[SFW]

Maybe for a short time. Ive done it, smoothie shakes for a week just to get enough protein in on a cal deficit.

But theres a reason why they call it a protein "supplement", because you are just supplementing your protein needs. Anyway, a true vegan would not be consuming whey. are you talking about soy protein? bad idea as your main source.