My rationale in this matter is basically one of semantics:
To me "Vtubing" consists of two concepts, which are similar but not synonymous and often overlap.
On one side you have the "mechanical" aspect of Vtubing, which is to say, the model. This is what most people use as the definition "A Vtuber is a content creator that uses a virtual avatar" but I'd argue this definition only refers to the MEDIUM of vtubing, trying to lump anyone who uses an avatar together under "Vtubing" is like lumping all of animation together because it's "animation". It's just a shared content creation medium.
On the other side I'd argue there's a much more important "Cultural" aspect to Vtubing, which is to say, the customs, unspoken rules, industry standards, language, visuals, attitudes and beliefs held by the majority (but not all) of content creators which call themselves vtubers. I'd argue that being a vtuber is more than just the model, there's a sub-culture involved and people in this industry, even if subconsciously or through trends, do indeed follow it. It's the reason why people often measure a person's involvement with Vtubing on their knowledge of our unique culture, do they know famous agencies or vtubers? Do they know what live2D is and its differences to 3D? Do they understand the concepts of graduations, debuts, generations and other slang?
In my opinion a true vtuber needs both of these, if you're just using the model as a gimmick or are totally unrelated to the medium and just get shoehorned in because you "technically" count then what's even the point? That's why imo vtubing began with kizuna, because she, along with others of that time, are the root source of what makes vtubing what it is. There is a very clear before, when random guys on youtube played with facerig and pretended to be a talking burger and after, when it was clearly a japanese niche sub-culture combining anime with idols and later streaming.
While I do think the first definition is correct, it is hardly satisfactory and often imo plays into why outsiders are so incapable of wrapping their heads around what vtubing is and why people like it.
