After importing the attachments thumbnails display correct but the file size for all attachments are listed as 0 and when you click on attachments i always get not found error both when SEO options are enabled or disabled
For some reason the field "file_size" in the database table "xf_attachment_data" has records value 0.
Just copy the field "filesize" from vbulletin table "attachment" to xenforo table "xf_attachment_data" field "file_size". It worked for me.
Vbulletin and xenforo are in the same database:
SET xf_attachment_data.file_size =
SELECT attachment.filesize FROM attachment
WHERE xf_attachment_data.upload_date = attachment.dateline LIMIT 1