Once the Danes (Viking is an old Briton word for "pirate") settled down they ceased being a threat. Part of that settling down was conversion to Christianity but it wasn't the whole in and of itself. Primarily it was the previous Danes that had settled into the lands they once raided and intermingled with the local populations, later Danes stopped raiding their own kin not to mention the all too frequent civil wars that raged in the Danish kingdom.The Vikings were indeed a formidable force, but once they converted to Christianity, they ceased being a threat.I guess Hadrian had no need to build a wall........ Oh wait.......
You should look at the Viking expansion even into what is now Russia........ Not many people were able to stand up to the Vikings.
The Saxons lost to the Normans based on two primary factors;
The Saxons were not very well organized.
The Normans had something the Saxons didn't, cavalry.