I’ve been browsing through an old notebook in which I kept notes about my Voynich studies lately. In June 2004 (boy, have we made progess since…!) I jotted down some ideas I had completely forgotten about by now:
“Apparently there are medieval ciphers where one vowel and the following letter are encoded with the same character: “a”/”b” -> <q>, “e”/”f” -> <r>, etc.
Of course, this would explain the occurence of triple glyphs.”
By “following” I meant “next in the alphabet.” Thus, the cipher would be basically a monoalphabetic subsitution cipher, but with “a” and “b” from the plaintext mapping to the same ciphertext character, “e” and “f” mapping to the same, etc.
Back then I devoted a bit of time and statistics to the issue, but aside of a suggestion of Italian as the plaintext language and a tentative mapping of EVA <e> to letters “i”/”l” (both “j” and “k” being uncommon at the time of creation of the VM, “l” would be the character following “i” in the alphabet), I didn’t get far.
Since 2004, I’ve moved away from simple substitution ciphers, because while the above scheme would indeed explain the occurence of three or more identical letters in a row, it fails to give an answer to the VM word grammar, why certain letter combinations are only ever found word-intitial or word-terminal, and other oddities.
Nevertheless, does one of my readers have more background where I may have come about this scheme?
Thoughts about the Voynich Manuscript 
Not an answer to your question, but a remark: triple (etc) glyphs may not be as frequent in Voynichese as they are in EVA. Something like “iiin” may well be a single glyph; in fact, its distribution seems to support it – “i” isn’t exactly a flexible unit.
There is a well-known medieval cipher where vowels are enciphered by the next letter along, ie a -> b, e -> f, i/j -> k, etc. This is known as the “magic(k)al cipher”, and was typically used to encipher spells, amulet inscriptions, etc.
It wasn’t secure (obviously), but the literature asserts that it was used more for magic than for secrecy.
Ah, so I had gotten it the wrong way around, it seems? — Thanks for the pointer!