f18v...Dioscorides...something about those leaves

[ReneZ]

For what it's worth, for now I would personally operate under the paradigm that the VM makers used a few different illustrated herbals for whatever it is they were doing. I would say there must have been a Dioscorides among those, but also at least one different type. 

I assume it may not be a stretch to say that a library that had one herbal MS may have had more of them.

The Visconti library in Pavia, which owned the MS now preserved as BNF Lat 6823, the "Manfredus" herbal in the Tractatus de Herbis tradition, also owned at least one copy of Dioscorides. This is a 14th century Latin MS, now BNF Lat 6821, which is not illustrated.

It also had a beautifully illustrated copy of the Tacuinum Sanitatis, but I am not sure right now which one.

We know this from:
Elisabeth PELLEGRIN. La bibliothèque des Visconti et des Sforza, ducs de Milan, au XVe siècle. Paris, 1955. (Publications de l'Institut de recherche et ďhistoire des textes, V.)
She edited a catalogue of the library made in 1426.

It does not seem to have had any copy of the Balneis Puteolanis.

All in all, this library could have been a great inspiration for our unknown autors/artists, was there at the right time and at a very interesting location, and was open to visitors.
However, it still lacks on a few important points.
Unfortunately, there are not so many catalogues of contemporary libraries, and this type of knowledge is extremely fragmentary.

This book has interesting details, but may only be visible online very partially:
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