All "mythical origins and legends surrounding " the VMs that I know of have the same degree of plausibility, that is, absolute zero. All of them are of modern make.
- I believe the first recorded legend is the presentation of the ms. by then-owner Georg Baresch to Athanasius Kircher, in a letter dated 1639, as a compendium of "treasures of Egyptian medicine" collected by some gentleman in his oriental "quest for true medicin" (quoted from Ph. Neal's translation, here; all my science about this & the following letter is from his Voynich site, here). The priority relies on 2 very plausible conditions: If the letter is authentic & if the document it describes is really the same book we dub the VMs.
Given that Baresch was striving to entice the famous author of the Prodromus Coptus into studying a copy of his ms., I cannot judge the extent of his sincerity in alleging an Egyptian origin. As an alchemist he must have known the stylistic conventions of the VMs do not match those of Hermetic diagrams in mediaeval mss, which would be all he'd ever met by way of hieroglyphs; on the other hand he may have interpreted some of the diagrams, as I did, as illustrating Moses' cosmogony (Gen. 1-2), and he must have recognized the Zodiacal signs & the (European) calendar months for what they are.
- Then, we have a letter from Marcus Marcii to the same Father Kircher (1665) translated here, asserting Emperor Rudolf owned the book before Baresch, having paid 600 ducats "to the man who presented it". Marcii then adds the opinion of a Dr. Raphael, "tutor of the King Ferdinand", who "thought the author was Roger Bacon the Englishman": the latter legend was widely circulated by W. Voynich in his promotional efforts for his ms. I think that contrary to us, the 17th C. did not question the attribution of De Mirabili Potestate Artis et Naturae & other alchemical treatises to Bacon.
Whoever received the 600 ducats is anyone's guess. The notion that it was John Dee has no other ground, I think, than a mention in his diaries that he obtained roughly this sum once, while in Prag. I am pretty sure he did not explicitize the reason for the payment. (Check this).
- Then, we have the claim by W. R. Newbold (publ. 1928) that, according to his partial decipherment, Roger Bacon had invented the telescope and drawn a schema of the Andromeda galaxy in the VMs; I think Jacques Bergier was the first to surmise in print that the anachronistic technology could be of extraterrestrial origin (Les livres maudits, 1971).
Some yrs ago I submitted to the VMs mailing list a much less, ah, aethereal conjecture: the diagram represents the Earth before the Biblical Creation. The supposed galactic bulge is actually the shapeless & void Earth; the arms, eight gusts of the Ruach Elohim blasting the Heaven and the waters above, off the tohu bohu & unto the eight compass points. Of course, I interpret the other diagrams of the folio as the 6 days of Creation, and the preceding folio as a map of Eden. In Newbold's defense, he likely had only photostat reprints of the VMs to work with, and may have missed the thin "T-in-O" signature of the pristine Earth: I recall it was not visible in my copy of Les livres maudits; I noticed it while browsing Mr. Davies' indispensable site, which all the links I provided to VMs pages h-ref.
- There is also a sidestream theory in Voynichology that will not slake in spite of its utter impracticality, which raises it to legendary status: that the whole thing was a fabrication by W. Voynich himself. The only factual basis of this legend is that Voynich was less than ingenuous in his account of the discovery of the ms.; it meets the same objections as the notion that the VMs script consists of glossolalia, compounded and amplified with the results of the radiocarbon dating, with his life-long inability to direct researchers to whatever prior art inspired him and, top and foremost, with his life-long inability to reach the desired end of the whole rigmarole: to sell the ms.