A name to attach to the origins of the manuscript

[Jorge_Stolfi]

Then again, a non-illustrated MS on paper would still have been a lot easier to make, and a lot less expensive.

Yeah, that is a good question: why did the Author chose to put the thing to vellum?

He must have had that material on paper first, either as unorganized notes or as a more organized paper book or set of booklets.  That should have been enough for his own use, if he had any.

But, a you say, he would not need the decorative illustrations, or made-up plant drawings.  

My guess is that he decided to make a vellum copy in order to "monetize" those personal notes and booklets, by selling them to some curious scholar, or collector of peculiar books. I imagine that a book on vellum would fetch a much higher price than the same contents on paper.  Surely more than the cost of the vellum.

That could also explain the echoes of Pharma in the Herbal.  I imagine him trying to sell the book without the Herbal, but being told "You call that a herbal? This is a herbal, see: it has one plant per page, with a full drawing of the plant, and at least a long paragraph of text..."  Then he going back home and starting to expand his Pharma notes into that format...

But maybe instead it was a vanity project -- he wanted to see his beloved notes set in a prestigious form, like other books that people treasured.

Or maybe he wanted to leave the book to his heirs, and thus chose vellum for its supposed durability.

All the best, --stolfi