The Voynich Manuscript is a cipher because...?

[-Job-]

Changing "The" to "De" isn't a hindrance, since the Anglo-Saxon and Norse "d" character with the bar or the Greek theta immediately come to mind for polyglots and its position in the word makes it a high candidate as a definite article, regardless of how it's spelled. The short "word" in front of the fairly short last word makes it unlikely that it's certain languages that put the verb at the end. Lots of clues.

The substitution of "the" with "de" would not remain consistent throughout the text, that's a key point. In my example i used both "the" and "de" to illustrate this, and would have certainly used additional variants in a longer text, with other substitutions as well as null characters (e.g. 'h').
You could find a way in via grammatical analysis (as you mentioned) and lots of trial and error, but that's assuming you're not wasting time with other alternatives.