The Vault ListCheckouts action enables you to query a vault repository for a report on all files currently checked out. The action can output the report as an xml file, or you can use the OnFileCheckout script event to process each file checked out by each user.
And to process each file checked out by each user, you can add code in the OnFileCheckout script event:
Setting the ActionResult to False will cause the action to fail after it has processed all of the files.