As far as I know, Scandinavia was absolutely a farming society to the same extent England and France were. It's not like they all up and went a-viking: the people that did were a small percentage of the population, and this is to be viewed as equivalent as Rome sending the army (a small percentage of Rome's population) to conquer Gaul. They aren't suddenly considered less of a farming society when they do this. It's just how farming societies fight each other.