By default, Vanta will set the reporter of the ticket to the user that connected the integration. If you wish to change the reporter, you will need to reconnect (not delete) the integration, and ensure that the integration is connected with a user that has the MODIFY_REPORTER permission in the Jira projects that you will be creating tasks in.
Prerequisites
Administrator access to Vanta
Admin permissions in Jira (including MODIFY_REPORTER)
Resolution
Sign in to Vanta and head to the Integrations page
Scroll down to Jira and click Manage, followed by Edit
Click the purple Connect Jira button
After following through with the connection, the integration should be reconnected, and you can assign the reporters in tasks created from Vanta.