Vendoring means you don't have to fetch the internet for every build, that you can work offline, that you're not at the mercy of the oh-so-close-99.999 availability, that it will keep on working in 10 years, and probably other advantages.
If your tooling can pull a dependency from the internet, it could certainly check if more recent version from a vendored one is available.
Is there any package manager incapable of working offline?
This is only true if you aren’t internally mirroring those packages.
Most places I’ve worked have Artifactory or something like it sitting between you and actual PyPI/npm/etc. As long as someone has pulled that version at some point before the internet goes out, it’ll continue to work after.