I dont see it here, so figured someone’s gotta post it. Here is the definition of veganism as made by the vegan society circa 1944:
“Veganism is a philosophy and way of living which seeks to exclude—as far as is possible and practicable—all forms of exploitation of, and cruelty to, animals for food, clothing or any other purpose; and by extension, promotes the development and use of animal-free alternatives for the benefit of animals, humans and the environment. In dietary terms it denotes the practice of dispensing with all products derived wholly or partly from animals.”
The difference here is you go with an idealist approach and the vegan society definition is more materialist.
This is not to say that ‘materialist is better than idealist’. The materialist approach starts from our current reality and built an idea from that (we exploit animals for many things, we should try to avoid that). I think it is way more precise and approachable to say that we try to avoid animal oppression as much as possible instead of starting a conversation about rights extension. This second approach can be difficult to apprehend at first and raises many questions.
Also in the materialist approach states clearly the reason behind the concept : unnecessary suffering. Talking about rights is 100% necessary too, but it can raise the question on why : then we have to talk about the consequences and the end goal.
Introducing vegan concept from a materialist approach seems slightly better to me. Unless you know your audience who specifically be more inclined to an other approach this is a good default.
Last point : I felt like you criticizing the vegan society here “vegan society was founded to promote than their own current definition”. But really isn’t this the whole point? Like anti-racist, women rights, trans rights, etc organizations, whenever you advocate for ideas you need to advocate for a specific definition of them.