With regard to the Scandinavians (the "Vikings" were a conglomeration of raiders throughout Scandinavia and really could have been anybody) it seems to be split genetically. Throughout Scandinavia the genetic haplogroups are pretty much divided between Haplogroup I1a and R1b. In the British Isles it's heavily skewed towards R1b with a very good showing of I1a, in Normandy it is about 50:50 split, in Sicily, which is the most fascinating of all the studies, R1b is slightly dominant to J2(which is considered to be the Haplogroup of the Mediterranean if you will) and I1a has a decent representation. In Ireland, R1b is the hotspot as it explodes in their population, but I1a has a representation, and Scotland might have a predominance of I1a. The Isle of Man is under represented in testing but I think they will be split as well.
So it is assumed by age of haplogroup that I1a is a north south migration as it does not appear east west with any frequency. R1b is a bit all over, but due to its high frequency in the USA and the bery large British Isles representation in the database, science has concluded that R1b is younger and emanates from an easterly place in Europe. This could all change as the database grows!!