How could you prove that VM is gibberish?

[Mark Knowles]

My best guess is that about 80% are meaningless and 20% are meaningful

80% of the text is bogus filler words? Why so much when fewer would have been more economical? So many more sheets of expensive parchment needed just to overbloat the manuscript with meaningless words?

The purpose of using filler words(stegnography) is to confuse the decryptor trying to break the cipher. The purpose of filler words is similar to the purpose of null characters/symbols. Fewer filler words would be more economical, but more filler words makes it harder for a decipherer to spot the real words. You are effectively questioning the purpose of stegnography. However stegnography was a recognised useful technique in the fifteenth century by cryptographers as we see in the references in Leon Battista Alberti's work and Trithemius's work.