Question: how to force a sub-package to be separated to another group?

Hello, first of all I would like to thank the author and maintainers for the excellent tool!

I have been using isort for years in a row now, but recently I came up with an interesting situation that I have never found before:

In one project I am vendoring an external dependency (because I need to apply some patches to it before using), so I have a project layout similar to the following:

.
└── src
    └── <mypkg>
        ├── __init__.py
        ├── ...
        └── _vendor
            ├── __init__.py
            ├── <deppkg>
            │   ├── LICENSE
            │   ├── ...
            │   └── __init__.py
            └── vendoring_instructions.rst

I would like to have my from ._vendor import ... import statements separated from the imports of the local package (from . import ...).

Is there any way of achieving that by changing isort's configuration?

I tried the following:

# .isort.cfg
[settings]
profile = black
known_first_party = <mypkg>
known_third_party = ._vendor
# ALSO TRIED: known_third_party = <mypkg>._vendor
# ALSO TRIED: known_third_party = _vendor

But that does not seem to pull of the trick (or maybe I am doing something wrong here).

Start with the .isort.cfg settings shown and read how known_first_party and known_third_party classify relative imports. Reproduce the example with the local package and its _vendor sub-package, then determine whether configuration can create the requested separation and clarify the supported behavior or documentation.