I don’t think these are json files, it’s json metadata.
It looks like vBulletin changed their encoding of attachment locations from
[attach]123[/attach]
to
[attach=json]{"data-attachmentid":123}[/attach]
and the importer cannot handle that. It should attach the attachments anyway, these tags are only for positioning them within the post. But the deletion of the tag only happens when they contain a numeric id.
A lot of other errors in the screenshot above are independent of this issue.