How could you prove that VM is gibberish?

[Rafal]

which shows that the initial and final word parts of gallows words are independent and likewise unlikely to be in a natural language

This is a good argument for me. Just like "autocitation" it supports meaningless Voynich hypothesis.

Of course there could be some encryption that destroys natural language features. For example simple substitution ciphers are easy to break because symbol frequencies are the same as letters frequencies in the natural text. More advanced ciphers equalize these frequencies.

And it could be, as Mark says, that Voynich is mostly meaningless but there is still some info hidden among the noise.