In one throw-away line, Johnson notes that some people have questioned whether the manuscript is as ancient as claimed:
The hoax hypothesis can’t be ruled out. But if the Voynich is a fake, it’s an elaborate one. A twentieth-century scam artist would have to have located a hundred and twenty sheets of blank six-hundred-year-old vellum in anticipation of the invention of radiocarbon dating (which did not yet exist when the manuscript first reëmerged, in 1912).This reasoning strikes me as naive. By the early twentieth century, some art forgers were certainly using antique materials (not in expectation of radiocarbon dating, but in expectation of trying to fool better trained experts), and the use of such old materials and old techniques was far more common in the 1920s. Eric Hebborn's Art Forger's Handbook, while slender on history, is very suggestive about the overlap between early art restoration and forgery--indeed some early 20th C restorers tried their hand at outright faking. (I'm ignoring the question of whether the manuscript's code is medieval or modern, which is a major buttress to it authenticity in Johnson's reasoning--but, frankly, that evidence could be argued either way, as the "Hoax" section of Wikipedia suggests.)
In terms of provenance, the descent sketched by Johnson has some gaps, and the overall provenance could go either way, but the romantic story about Rudolph II makes me a bit suspicious. It sounds a little too good (to be true) and while the story might have been tarted up by Voynich to sell the manuscript, it wouldn't surprise me that an anonymous forger used the story and references to create this minor masterpiece that still excites the fancy of writers and laypeople.
The Unread: The Mystery of the Voynich Manuscript (New Yorker, July 9, 2013)
Update 2/20/2014: Professor of Applied Linguistics, Stephen Bax, at the University of Bedfordshire, claims to have made a start deciphering some of the coded language by comparing names and known images to medieval Arab manuscripts. From a university press release:
Although Professor Bax’s decoding is still only partial, it has generated a lot of excitement in the world of codebreaking and linguistics because it could prove a crucial breakthrough for an eventual full decipherment.
“My aim in reporting on my findings at this stage is to encourage other linguists to work with me to decode the whole script using the same approach, though it still won’t be easy. That way we can finally understand what the mysterious authors were trying to tell us,” he added.
“But already my research shows conclusively that the manuscript is not a hoax, as some have claimed, and is probably a treatise on nature, perhaps in a Near Eastern or Asian language.” [emphasis added]Obviously, this is preliminary and will probably be altered as time goes on. -PWR
(update: fixed some typos. Update 10/14/2013 update for comments)
(update 2/20/2014: Professor Bax news)