I am not convinced by the provenance history of the VMS

[Jorge_Stolfi]

The 1666 Marci letter was literally found inside the manuscript when it was opened by Voynich.

Because of the presence of the Marci letter ... we can be confident about ... the fact that it was sent to Kircher.

AFAIK there is no evidence that the letter was inside or attached to the VMS when Wilfrid acquired it.  In fact there is no mention of the letter anywhere before Wilfrid's announcement.  

The only possible mention of the VMS between 1665 and 1912 is a very short entry of an "anonymous 15th century MS" in a list of books compiled by the Jesuits sometime before 1912.  That item can be identified as the VMS only if one assumes that the VMS must be on that list.  

But that entry does not mention any attached letter.

These aspects are so clear, that only conspiracy theorists tend to doubt them.

A conspiracy is just an agreement by two or more people to do something in secret.  The popular term "conspiracy theory" means a claim of a conspiracy by thousands of people, like all airline pilots or all scientists at NASA.  Such claims are ridiculous because such a large conspiracy would be hard to set up and impossible to hide.  

But conspiracies do happen.  Even conspiracies involving dozens of people. Ask Julius Caesar...  

The various claims that the standard VMS history is wrong and/or the actual history may involve some kind of fraud would require conspiracy by very few people; say, Wilfrid and a couple of Jesuits.  (And in fact we know that they did conspire -- to arrange the illegal sale and keep it secret.) Or the fraud may have been committed by Wilfrid alone (that is, it would not be a conspiracy at all).  Or the claim may require only accidental errors rather than fraud; like the letter getting separated from the original book and getting attached to the VMS by mistake.

[Marci and Baresch] were trying to pique the interest of Kircher, an otherwise extremely busy man, to have a look at deciphering the book. This is the reason for them claiming it's probably medical. They wanted to create the impression of a book containing vital knowledge which it was almost Kircher's duty (as a priest and a man of science) to decipher.

That is a weird conspiracy theory   .  Marci had no intention of piquing the interest of Kircher; he sent the book because he knew that Kircher had asked for it.  As for Baresch, he said it was probably medical because it obviously looks (and is) medical.  And that is apparently why Baresch was so interested in it.

All the best, --stolfi