On the off-chance when this comes up again, generally a way to really get a response from a group (whether a company or not) when they've become non-communicative is to have at letter that at least looks like it was written by a lawyer citing their non-communication as a sign the company is de facto dissolved and that all contracts will be treated as null & void after 30 days.
Something along those lines will always force an issue, because it doesn't matter how incompetent the other side is. The instant you threaten to take something, you'll get a response if they're actually there. In the case of a vtuber, I'd start filing some copyright protections over the IP in the local jurisdiction. IPs can be acquired in many ways, and its a 2-way street.