When I read Vellum a couple of years ago, I had mixed feelings about it. After careful consideration and some digestion I decided to give Vellum four stars, because it's quite an interesting book.
Vellum is a challenging and different kind of book. It isn't a linear book, so it must be read with thought. That's why some readers will love this book and others will hate it. I personally like Vellum, because it's something different and it's quite an ambitious book (there aren't many writers who can write this kind of books).
I can recommend Vellum to readers, who enjoy reading challenging books. It's a unique reading experience, so don't be afraid to read it.