How were they depicted by the Romans in Ancient History?
Now, the Roman empire from antiquity never encountered what is usually called vikings: sea-born raiders from Scandinavia (the earliest recorded raid that could plausibly be said to be done by vikings were the one lead by Hydelac, known from Beowulf, which is also attested by Gregory of Tours in his Historia Francorum, but that is one hundred years after what is usually considered the end of the western Roman empire), but the eastern successor, based in Constantinople, certainly did (at least if we stretch the meaning of "viking" a little bit).
The Varangian guard was an elite force composed of Scandinavians (and later Anglo-Saxons), serving as bodyguards for the emperor (foreigners have few previous contacts in their new environment, and are dependant on the goodwill of the ruler, which lessens the risk for betrayal). They also served as an reserve force in many battles.
Later Norwegian king Harald Hårdråda was perhaps the most famous Scandinavian to serve in the Varangian guard, where he is reported to have risen in rank, but also to have been imprisoned on charges of misappropriation of plunder.