Your question brings me back to the days when I first started studying Latin in earnest. I purchased copies of the vulgate, the Clementina, the Stuttgart edition, and the Nova. I started reading the Vetus Testamentum, but pretty soon I figured out that the Novum was (usually) a lot easier.
This was thirty years ago, and my Latin has improved enough that I really don't find any part of the Vulgate very hard any more. But since you asked, let me bore you with some of the vague memories of my youth.
My recollection is that the Gospel of St John was the easiest, followed (fairly closely) by his three epistles.
Next, came the three synoptics. They were pretty similar in difficulty, but my fuzzy memories say Matthew was a bit easier, and Mark a bit harder.
Next came the epistles of St Peter.
After that in difficulty came the epistles of St Paul. Hebrews being the easiest, and the Pastorals being the hardest.
Then the Apocalypse.
Then Jude and James.
Then, a distant last, Acts. Indeed, I had found no book of the vulgate more difficult than Acts, although I have only read sporadically from the Vetus Testamentum.
There you have it. I was an American, born in the sixties, educated in public schools, who, on a lark, devoured Gavin Betts' Teach Yourself Latin, and bought myself a Chambers-Murray dictionary, and a preposterous slew of vulgates, and had at it. It was a very fun slog. Since that does not describe you (probably) your mileage will vary.