It seems like it depends on how the authors have configured Vouch. They might completely close the project except to those on the vouch list (other than viewing the repo, which seems always implied).

Alternatively they might keep some things open (issues, discussions) while requiring a vouch for PRs. Then, if folks want to get vouched, they can ask for that in discussions. Or maybe you need to ask via email. Or contact maintainers via Discord. It could be anything. Linux isn't developed on GitHub, so how do you submit changes there? Well you do so by following the norms and channels which the project makes visible. Same with Vouch.