Vodoun is primarily an oral tradition, which means there is no orthodoxy of spelling or pronunciation. Secondly, Voodoo is practiced in many parts of the world, and each practice has been influenced by native languages and habits. Names and terms relating to Vodoun are a combination of African, French, Creole, and so on, so you might see many spellings of one name, or many names for one thing. Usually, when you see it spelled "Voodoo," you can assume Western, US practice; "Vodoun" is typically reserved for more traditional West African practice.