Cardan grille or substitution cipher

[davidjackson]

It's a method to select certain letters from a block of text. Although there are many forms, it's basically a cut out template you put over the page, and so select certain letters.
The rest of the letters are just nulls and can be ignored.
If you wanted to increase security, you could apply some sort of cipher (ie substitution) to the text before you surround your letters with the nulls. But there is no point in putting the nulls through the cipher.
I'm not saying this is how the Voynich works, just commenting on your original question.