G Suite - Access to Vault for external user

Is it possible to allow external users such as members of a regulatory agency access to my organization’s mails via Google Vault?

Yes, that’s possible by adding the users to your domain in the Admin console and giving them rights to search in Vault, view reports or other tasks. So the external user can get access to your archived files like mails. It’s also possible for him/her to see all details and activities of communication such as changes and storage dates. You can find more information about this topic in Google’s support article.