How perfect is Voynichese?

[Jorge_Stolfi]

Uh, no, not even close. A one-time pad is theorethically perfect, it's way harder to decrypt even than, say, RSA encryption.

For a letter-size document, perhaps.  A book-size text in codebook cipher can be cracked by guessing the most common words and using external clues (topic, illustrations, recurring formulas, partially leaked plaintext...) 

The Rohonc codex seems to be an example of a "codebook" cipher (and even in an orignal language) that was cracked this way.

All the best, --stolfi