Your organization can use Vault QMS External Response Collaboration feature set to allow external participants to be a part of the Investigation process. They can receive tasks with due dates (and the associated notifications), and provide their responses to field prompts directly with the native Vault experience. The external user can even directly enter the Root Cause and Summary of investigations in Vault. This enables faster close-out and mitigates the need for, and risks associated with, manual re-entry of responses received from partners. The specifics of what the external user can and cannot edit on your Investigation records is fully configurable.
An example configuration might make your process for getting responses on Investigations look something like this: