Vtuber unions is a dumb idea. Unlike VAs, vtubing is a "profession" where your work is almost entirely your own responsibility. You can't blame not being given a role for not being able to work for example.
If you have the clout or talent to become a big vtuber then yes, you don't need a corporation - start your own company with 1 or 2 employees to help you with shit. Otherwise hire the services of an agency with a manager or something to deal with business related tasks. If you want the support or structure a corporation gives then you're gonna have to pay for it one way or another but plenty of successful indies show that it isn't necessary.