A big issue in this critique is the fact that, often times, virtual youtuber as a tag has its use as a context clue - a lot of uploads which don't have their character tagged are basically indistinguishable from other posts that could be found under tachi-e, and often too these uploads may not even have their commentary included (which may or may not explain that this is a VTuber with social media links), so for someone trying to tag these characters, the tag is one of the few context clues provided to aid in tagging. This isn't even mentioning fictional VTubers, such as in post #3004550, post #3055431, post #3464101, post #3844473, post #3944707, etc; or VTuber concepts/designs which may not even have names, such as those found in Pixiv's Virtual YouTuber audition, Mirai Akari's New Virtual YouTuber Illustration Contest or the 4chan VTuber Competition.
But you aren't necessarily wrong in terms of its usage, heck, it's been referred to as a generic-copyright tag. But that's strictly a consequence of the universal nature of VTubers, in contrast to other profession tags.
However, having a tag for the act of VTubing, now that is a valid point to raise. There are a number of posts which do reference this act, but an issue is that, depending on whom they are referencing, it'll either be a livestream or a video. In terms of livestreams, there are those examples you named, as well as post #3770049, post #3365164 and post #3077457 (rating:e), but then you've got in terms of videos posts like post #3422857, pool #13572, and so on.
Having utility aliases for both livestream and fake video would probably be a good idea, same with having both linked in appropriate wikis!