Well, first and foremost obviously the docs of the DocumenterVitepress.jl 
Now, the documentation of the Documenter.jl itself, in the ideal case, should
- Illustrate how the standard no-frills Documenter-produced docs look like. That is what IMO >90% package authors are interested in and would be happy with,
- Showcase some special options,
- Well, actually document the package.
For the first two items, the Documenter.jl docs should be produced with Documenter.jl itself. IMO it would be very strange to have it done with some additional package.
All these three items are equally subjective. Actually I find the Documenter navigation easier, also because it is consistent between many packages and the docs of Julia itself.
As for advertising a package: The docs of e.g. Makie.jl should be beautiful, if possible. The docs of Documenter.jl could better be modest and functional.
In short: Maybe I’m misunderstanding or overlooking something, but as I see it now, I’d strongly prefer for Documenter.jl docs to stay as they are, i.e. produced by the package itself.