"Vendor packages" are npm packages that match any glob in the "vendor" array (for more info, see "Configuration"). These packages never have their dependencies installed (or their build scripts executed) when running the indo command. But they are still linked to as dependencies of non-vendor packages.
This is useful when working on other dependencies that don't belong in the commit history of your monorepo (not even in its .indo.json file). For example, you could be developing a new feature in a dependency and testing your changes within your monorepo.
To use vendor packages, use git clone or ln -s inside the ./vendor/ directory to add your repositories. When all of your clones are ready, run the indo command to link them to your non-vendor packages. It's that easy!
Another option is to use indo link. More info here.
You can even clone a monorepo into the ./vendor/ directory. Right now, only Yarn workspaces are supported, but Lerna support will be added eventually. The indo command uses the "workspaces" glob array in each monorepo's package.json file to find more vendor packages.
Unfortunately, a vendor package cannot be automatically linked to another vendor package, as this would break the promise that we never touch the dependencies of vendor packages.
As a workaround, you'll have to link the vendor packages together manually. If the vendor package is a symlink leading outside the monorepo, you should clone it before making changes, so you don't break other monorepos linked to the vendor package (if that's a concern).