Well, although black metal is the metal genre I listen to the most I must admit that I do not know much about viking metal either. As far as I know viking metal is black metal just with all this viking image, because some guys thought it would be cool to sing about and dress up like vikings and say they're the most real Norwegians around and shit like that and then everybody was like "Hey, their music is cool, let's be like them" and now lots of people do this shit. As far as I know the music is basically the same, but sometimes has more acoustic stuff - STILL as far as I know. Enslaved are categorized as viking metal as far as I know, but they're pretty brutal, at least "Mardraum" is. Bathory's "Blood on ice" is also labeled viking metal and is far, far different! Lots of acoustic guitars and clean vocals and really not much harder than grunge and rock stuff!