what is your personal theory on what the VM is?

[Mark Knowles]

If it was kept by one family member other family members could see it when they visited that family member if they wished to.

If the VMS should have been the work of a family and always in family possession, then the question arises about the sense of the "encoding". This could be explained at best with the assumption that one did it simply because one could do it. In any case, access to the VMS, i.e. to potentially explosive content, from outside does not seem to be a problem in this scenario.

You are right the central question about the Voynich manuscript is thie "encoding" and most importantly that we can't yet decipher this encoding. Without the encoding it would very likely be an intriguing medieval manuscript amongst many other intriguing manuscripts. It is the fact that we can't read it that really makes it stand out. I would say the Voynich mystery is the "encoding" everything else is details. We are left with the questions:

Why was it encoded?
Why can't we decipher the encoding?
Why aren't there any other manuscripts from around the time that are encoded in the same way?

In essence I think your statement that "This could be explained at best with the assumption that one did it simply because one could do it." has a lot of truth in it. However I would qualify this statement somewhat. I think one, or more than one, of the authors had already had significant experience in cryptography years before any of them even thought to write the Voynich manuscript. So I don't think a cipher was invented as it is felt necessary to keep the contents of the Voynich secret, though there may have been a small element of that. I think rather the Voynich was written in cipher as it was an opportunity to experiment with a complex cipher by someone(some people) who already had a strong interest in and knowledge of ciphers. I think the cipher is really part of the work. The manuscript is a work about herbs, astrology, cosmology and cryptography i.e. a rounded scientific text of the time covering various area of what they thought of as "science" of the time. The closest parallel from the time that I can think of is the work if Giovanni Fontana. Another loose parallel I would think is the work of Abbot Trithemius.

As I said before what I have written here is speculation.