Vegans in general choose to not consume animals or (non-human) animal products, by definition. There is a large variety of adherence and some variety in self-definition, just as there is in any common human community.
The Nirvana Fallacy lies in claiming that all vegans are vegan because they wish to be perfectly ethical, or follow a particular definition strictly, and are therefore failing on their own terms.
But vegans in general make no such claim to be perfectly ethical. They are not failing on their own terms - they are failing on your invented terms: ones which only highlight your own cognitive dissonance.
Veganism, however, has strong ethical foundations, and any amount of research shows this.
Unless we are considering the mental health benefit that comes with feeling ethically superior by being vegan, there are no valid health conditions that support unilateral veganism. It's akin to swearing off all liquids because of a lactose intolerance when consuming milk.
Veganism relies on the assumption that commodification of animals and products created by animals is considered unethical and should be rejected. I don't disagree with the sentiment to a degree, but I also don't agree with it absolutely. I do feel the need to point out that there is a Nirvana Fallacy within veganism itself, as it fails to truly define what is considered an "animal", and what is considered "commodification", and this is where conversations frequently turn into splitting hairs. Many animals (humans and not) engage in symbiotic relationships. Dogs will guard a home, and as a result, will be fed and protected by people in that home. It's how families work, friends, etc.
If a sheep sheds wood, naturally, is it anti-vegan to use that wool to create a coat? What about skinning a dead cow (natural causes) for its leather? There's a ton of gray area.
Veganism, at face value, is an idealistic platitude based on a beautiful notion, but it doesn't work on its own. There are a lot of great things about it, but it has never held much sway in my mind besides, "That's a cool idea, and I like seeing strides being made to make it easier to make 'vegan' choices, but I cannot bring myself to promote it".
