How quickly did the VMS leave the possession of those who understood it?

[Bernd]

The hypothesis I'm working on is that the manuscript was originally meant to be an unbound stack of bifolia, rather than nested quires. As the original poster noted, there is significant evidence that the current structure is a mis-binding:
1) mixing of scribal bifolia within quires;
2) the multiple examples of illustrations on a verso crossing the gutter to emerge on the recto of the non-consecutive conjoint (e.g. the two streams that cross from 78v to 81r);
3) the mixing of pharmaceutical and herbal bifolia in quires 15 and the two out-of-place herbal bifolia of quire 17;
4) the data analytics conducted by Colin Layfield and myself (explained in my Toronto lecture and hopefully soon to be published)
...among other evidence. But there is also no evidence of a complete re-sewing, which means this is the first and only sewing structure, which means in turn that the first binding was itself the mis-binding. (again, explained in my Toronto lecture)
The sewing structure is fifteenth century - three sewing stations of double-cords with the extended endband structure that developed in the fifteenth century. The quire numbers and the month names are (likely) fifteenth century as well.
Putting it all together, we end up with a manuscript from ca. 1420 whose creators didn't intend for it to be bound. Later in the 15th century, someone who didn't understand the manuscript but wanted to preserve it created the nested structure we see today and secured the quires between wooden boards, which is when the quire numbers would have been added. 
The current limp vellum covers are nineteenth century, but it was only the covers that were replaced then...the sewing structure remained intact, except for a few alterations during various conservation treatments in the 20th century. Rene has shown that the current covers are in the style of other manuscripts and books bound by the Jesuits in Rome, so it seems likely that the VMS was re-covered by them as well. The 19th-c. paper spine-liners (tiny scraps of paper adhered to the outside of the quires, between the sewing stations) support this idea.
To answer the original question, then, if my work is correct, the manuscript had left the possession of those who created it by the time it was bound, sometime in the fifteenth century. Unfortunately, there is no real way to know precisely when that happened...could have been 5 years later, could have been 50 years later, but definitely by ca. 1500.

I think a plausible scenario is that upon death of the author(s), the bifolia got into someone else's possession who did not understand what was going on but for obvious reasons found them interesting / potentially profitable. So he had them (mis)bound to a book and sold the resulting manuscript.

I do not believe the VM was intentionally created as a hoax. Wouldn't you rather create and bind a book than a large number of loose bifolia? Also the quality of drawings and text is too inconsistent, the whole corpus is unnecessarily large and detailed. And think of all the weird properties of Voynichese. Cui bono? Nobody in the 15th century would have noticed most of them. You could easily get away with a fraction of the work. This means the bifolia likely had have been of personal importance for the people who created them, for whatever reason. They probably were not intended for a larger audience hence the meaning likely died with its creator(s)