Well, they did call the Germanic tribes barbarians but the same name stuck for the Vikings. After the conquest of the northern parts of Europe by the Romans, they would have encountered the occasional raid from the Vikings. This is due to the fact that every winter the Vikings would stay at their villages but would go out every spring and fall on raiding seasons. On these seasons they went south because they believed that the world ended if they went to the west. The Romans would have encountered the Vikings during these skirmishes and would find more of them later one when they took to 'civilizing' Britannia. Of course, this meant that the Romans fought a war against the clans of Britannia and the Vikings, who had happened to have pacts with a clan or two. 

The shield wall would most likely come later on. That move was essentially created by the Spartans and adopted by the Romans because it was a great move. The turtle is also another Roman move which clarified that all shields were formed into basically a turtle. The turtle was based off of the shield wall which was purely kept between the Romans and the assimilated societies of Greece. It is unlikely that we would be capable of having knowledge of it unless we are set in the period of time after contact between Vikings and Romans. Essentially, the Vikings assimilated the shield wall after several battles with the Romans.