How could you prove that VM is gibberish?

[oshfdk]

So is it possible at all to say 100% sure if the text is meaningless and not in some language or encryption that we don't know?

I think there is no practical way of doing this. This is why it's so easy to blanket dismiss all meaningless hoax theories as "mere conjectures". Unless you find an undeniably original signed affidavit from the author of the manuscript describing its specific characteristics in great detail and providing a reasonable account of why the meaningless manuscript was created, I don't think there ever will be a sufficiently strong argument for any meaningless MS theory. Even if the text was generated using some stochastic approach (and we reverse engineer this approach), all it shows is that a manuscript like this can be created using this approach, not that this manuscript was definitely created using this approach.