The Book of Vile Darkness has been a disappointment on so many levels. The logic throughout the majority of the book is so corrupted by false premises and assumptions that it has created a world where there are no good guys and for a game built on heroes that is simply untenable. The gods are mostly useless and often so poorly written that it makes my head spin. The feats are either game breaking, redundant, or just an awful mess. The majority of the spells are reworkings of previously existing material (I'm looking at you No Light). The monsters are, again, from mostly previously existing material. And even Chapter 7 Lords of Evil, which is the most interesting chapter in the entire book, is nothing more than a re-hash of previously published material.
I can not understand why anyone would ever recommend purchasing this book after they've read it. Instead you're far better off answering the following questions and building your campaign's evil forces around them.
1.) Who are the major evil powers in your campaign?
2.) What makes them evil?
3.) Who and what works for them?
4.) What are their long term plans for your campaign world?
5.) Who or what is the most powerful evil entity in your campaign?
6.) What are their long term plans for your world?
Now if you're looking to find some really good evil gods to pepper into your campaign then you should look at the Monster Mythology. This book has some amazingly detailed gods that can be slipped in most anywhere and that are fully compatible with most any Dungeons and Dragons style campaign.