You may or may not have a Table Prefix, that depends upon the database, but vBulletin should take care of that, this is how all of the vBulletin code works.
The db code you mentioned above adds the thumbnailurl column to the thread table, which I can see from the image does already exist. The $threadinfo doesn't reference the table itself, but a PHP variable that's defined to hold information retrieved from the thread table.
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