While I do not have an answer to your ongoing question, I happened to come across a similar subdomain offering which supports Vercel out of the box:
All of this really feels like a grey area and that it’s bound to receive a suspension from GitHub. I remember seeing a similar domain offering on GitHub and the project ended up being suspended. Although this one in particular is supported by Cloudflare’s Project Alexandria (as seen on the footer of the project README) and I don’t know how Vercel treats these free domains, either. I also don’t know whether the domain having a .dev TLD has a higher decision-making power in suspensions.
Better if you could get a 1.111B Class .xyz Domain for a dollar an year: