When using the Covenant Extended Rules for Book Qualities, it is not explicitly stated but the rules seem to imply that the quality of a copied book can be modified by the skills of the copier. Examples include Exemplars, Flap-bound books and Juvenilia all of which are poor quality copies which are then used later to make better quality copies.
So if the typical sound tractatus is 11, presumably that is because it has Professional Scribes, Illumination and Book Binding. If someone with less than 6 in those three scores copied the book then it would presumably drop to Quality 8 which would be vain quality.
Could not a covenant buy a vain tractatus and then have professional scribes, illumanators and book binders copy the vain tractatus giving it a +3 to the original quality?