Using vcs repo without composer.json

Hi,
I have successfully installed wordpress, defined in composer.json as *package* - pointing to a zip with a specific version.

As far as I know it is not possible to reference a Git(Hub) **vcs** type repository, without composer.json file. It could be done by providing missing metadata in my json file, therefore when upgrading, I would not need to edit the repository information (as is case with the *package* type).

Is there any different issue that would arise? I see that composer can already convert git tag to a version number. Thanks