The Vikings are known for their glorification of violence and brutality, and yet the modern inhabitants of Scandinavian routinely score towards the top of the Global Peace Index.
I realize the amount of time between the two societies, but is there anything of interest that could have been responsible for this, such as an exodus of warriors into conquered lands, or is it pure coincidence?
Well it is worth noting that the reputation of the Vikings as a particularly violent or brutal people is vastly exaggerated.
All of our written sources on Viking activity in the 8th, 9th, and 10th century are written by "victims" (largely members of the clergy) who viewed the Vikings through an understandably biased lens. Because of this the nature of the violence and the descriptions we have are heavily colored. These images of barbarity only increase over time so that 12th and 13th century accounts of 8th century Viking attacks become far more violent and sensational than accounts from the time period itself. In essence the reputation of the Vikings has as much to do with "bad press" as with any particular activities or behavior.
This is not to say that Vikings didn't engage in warfare, raiding, pillaging, etc. But there is in fact nothing exceptional about their behavior when compared to other medieval peoples.
The Vikings in no way have a monopoly on violence or brutality. What they happen to have is an "advantage" during the "Viking Period" based on a variety of social and political factors both in Scandinavia and in Francia and the British isles that makes them the aggressors during this period. When we look at the sources we can easily find examples of Franks, Anglo-Saxons etc engaged in brutal behavior, raiding Saxon villages, taking Slavic slaves, killing kinsmen in feuds, blinding or mutilating each other etc. As Timothy Reuter put it
We have heard much about the destructive effects of the Vikings on Frankish society in the ninth century: we forget that for most of Europe in the eighth and ninth century it was the Franks who were the Vikings... [Timothy Reuter, "Plunder and Tribute in the Carolingian Empire" in Transactions of the Royal Historical Society, v. 35 (1985), p. 91]
So all of this is to say that we really can't compare a manufactured "Viking" reputation to modern "peacefulness". As you yourself point out there is a huge amount of time between the Viking Era and the modern period, some 1000+ years of development social, cultural, and political exist between the two.
As to why Scandinavia is now rated so high on the Global Peace Index, you'd probably want to talk to a modernist and even more probably to a Sociologist (perhaps over at r/AskSocialScience ).
I hope that clears up some things.
For some reading on the issue of violence and the Vikings here are a few articles of particular value in addition to Retuer's article which I cited above.
Coupland, Simon. “The Vikings on the Continent in Myth and History.” History: The Journal of the Historical Association 88, no. 290 (2003): 186–203. This article examines the ways in which common ideas about Vikings, including their propensity for rape and pillage, were created and how they compare to the actual documentary record.
Halsall, Guy. “Playing by Whose Rules? A Further Look at Viking Atrocity in the Ninth Century.” Medieval History 2, no. 2 (1992): 2–12. This article examines the propensity for violence across the board in the early medieval world and how Viking activites fit into this sort of system.
Sweden has had 200 years of peace, following three centuries of near constant war and many wars in the 4 centuries from the end of the Viking Age to the start of the 16th century. It was one of the most bellicose nations around during its imperial period.
Similarily, Denmark (Denmark-Norway prior to 1814) fought dozens of wars as well until what might be argued as it's last 'real' war being the Second Schleswig war in 1864.
This is not counting the occupation of Denmark and Norway in WWII and their NATO engagements (of which there have been a number) since. Iceland (should it count) is also a NATO member.
Finland - if they're to be included in Scandinavia (geographically they're not, but the term often refers to them as well) - has fought 2-5 wars since its independence a century ago, including a bloody civil war.
The country with the most peaceful reputation is Sweden, for their neutrality in most conflicts in the last two centuries, and during WWII in particular. But that is a 'recent' thing in the context of the 1000 years that have elapsed since the Viking Age ended. The Viking Age has no relevance to it.