Was the VM a failure?

[Bernd]

I would assume abstraction rather than deception here. It was probably intended that the plants in the plant section, for example, cannot be assigned to real plants with certainty. Nevertheless, the first section is a plant section as can be seen in many other manuscripts. This is true even if the plants in the VMS cannot be clearly assigned to any known tradition. It seems as if the author wanted to prevent plant names from being deduced from the coded text ( usually first "word" ? ) at any point.

What is the use of a plant illustration if it won't let the reader identify the plant?

It is a very common modern misconception that illustrations are used to clarify things.
Certainly, that is the case nowadays.

Especially in early herbals this was NOT the case. Minta Collins is very clear about this. Highly recommended reading.

Initially, this was done to beautify the book, or make it more interesting. The people making the drawings were not the physicians who actually knew what the plant looked like. They did not even realise what are the plant properties that help to identify them. 

This last point was included in one classical work, by Theophrastus. This was only intruduced in Europe in Padua around the time of the Voynich MS creation, but few people could read the Greek. Translations appeared later, and plant drawings that were good enough to recognise the plant started to appear in the 16th century in the times of Brunfels, Bock and Fuchs.

I agree with Rene.
As I have already mentioned before, we must keep in mind that almost all of the beautiful antique and medieval herbals still existing today were never intended to be used by physicians. They were high-end collectibles created for a bibliophile noble audience, hence lavishly illustrated, but barely touched. No physician could afford such a book. The original Dioscorides was completely unillustrated. The high-level books were also not copied by people with medial knowledge but compiled in workshops by artisans. Independently from text scrolls and stock image paintings. This frequently led to compilations of different plants and mix-ups in imagery - which didn't matter in the end. Because the buyer usually had no medical knowledge either.

Furthermore, no physician went out to collect herbs. At least none that could afford an illustrated book. This was considered an inferior job for women. A physician diagnosed patients and suggested a remedy. Ingredients were usually bought, prepared and administered by others. The only case where the plant's appearance mattered to a physician was to deduct potential uses from the shape on a philosophical ground. I would also not exclude a mnemonic component, especially if we look at the creative root shapes in many illustrations.

Yet many herbals show a consistent display of plants and imagery was conserved from antiquity into the middle ages. Likely by durable paintings on wood (pinakes). The VM plants are odd in many regards though they definitely are inspired by different known sources of herbal imagery like Dioscorides and Tacuinum Sanitatis. What really sets them apart are the flowers. Not only are they weird and most likely original creations of the artist, they are also unusually large compared to most other herbals that put little emphasis on flowers.

I cannot say what any of this means, but it is unlikely that the VM was used as a physician's toolbook. Such must have existed but probably were sparsely illustrated. illustrations=space=parchment=money. The VM must have consisted of around 250 pages including the now missing folios, most of which were occupied with more illustrations than text. Not a very economical approach. But all of that ends in unfounded speculations.

I stand by my point that the VM wasn't a success story that was ever repeated. And we must ask ourselves why.