Actually not often as we thought @ronavel
But only when they get into a Raiding mood and that's when they dust off the fancy Drakkar Dragon Longboats put on their best horns and Viking wear and row their boats towards their target. With their fierce reputation the sight of their ships are enough for most villages to surrender immediately and give the Vikings what ever plunder they could carry. The Vikings are pretty good at assessing their target. If the village is too well defended they would bring goods to trade in their Knarr ships. Otherwise....ARRR!(With a Norse accent)
Therefore their Method of Operation is remarkably similar to Pirates.