Anytime you talk about witches, witchcraft or anything similar, sooner or later the topic of voodoo is going to come up. In fact I have brought it up many times this month already.
The most recent season of American Horror Story: Coven did a great job of setting up Witchcraft and Voodoo as rival factions of magic-use. Personally I loved it and thought it was well done.
What people typically think of as Voodoo comes to us from Louisiana Voodoo. Which, one could argue, has a lot of theatrics to help in the tourism trade, but has a religious practice as well. The religion part of Voodoo is more similar to what is known as Haitian Vodou. There is a third type that I call Hollywood Voodoo which is what American most often see. It raises zombies and is more like what we saw in AHS:C and the Anita Blake books.
In the WitchCraft RPG by C.J. Carella we see voodoo, or Voudun, as another group of the Gifted. Pretty cool given his work on Voodoo The Shadow War for GURPS. In other games voodoo is used as a subset or something akin to witchcraft.
Voodoo with/as/vs Witchcraft works great in a modern game.
It's games like D&D and Pathfinder where it is a bit more difficult.
Voodoo (and by this I mean the "Hollywood" version of Voodoo) works because there is such a great interplay between old-world African gods and not-as old Christianity. Your typical FRPG does not have this really. One could create a couple of pantheons of gods and have the place where they intersect work like voodoo, but there is no reason that they have to work exactly like voodoo at all. In fact in most games what works well enough is have a priest.
When I first opted to include voodoo with witches it was with a huge grain of salt and caveats. I wanted to include it because I really liked the idea of having a voodoo witch. I also had a great character called Bone Man who was a voodoo priest and I really wanted to play him in the rules I was writing.
For my new book, Strange Brew, I still not have 100% decided on whether or not Voodoo will be a witch archetype/tradition, a new cleric archetype or even something else entirely. The Loa are really similar to the idea of Patrons to be honest. After all, Papa Legba and Baron Samedi are too cool not to include.
It will give me a great excuse to finally get Bone Man written up.