And where is the proof of their achieving this en masse? Vikings were rudimentary Scandinavian raiders that attacked coastal European villages (so basically fishing towns) using hit-and-run tactics like pirates and brigands. They never defeated or challenged British and Germanic tribe interiors en masse. There's plenty of revisionism in what people think of the Vikings. The Romans did more to England, Scotland, and the like than the Vikings. The same Germanic tribes you're talking about came to defeat the Romans. When the British marshalled their strength, the Vikings never came again. I haven't seen much evidence Vikings were superior fighters than the, say, Ottoman.
Wait, what? Have you just made the rookie mistake of assuming that because people stop talking about "Vikings", "Northern Horde" and "Heathen Army" that the Vikings died out?
The Vikings did not die out.
They became the rulers of major tracts of England and France and were eventually just known as Normans, English or French. Plenty of English towns, especially in the North and East Midlands still carry Viking names. The entire region they settled in France is still named for them - Normandy.
They settled France so quickly, effectively and permanently that it only took a few generations before they were just regarded as French. In 1066 William The Conqueror made The Normans the last foreign power to successfully invade England, taking the crown from the last Anglo-Saxon King and keeping it "in the family" arguably until the Wars of the Roses, nearly 400 years later - the Romans never got close to that in England. Anglo Saxons were never to rule England again after William's Vikings arrived for good.
The Vikings are one of the most successful conquering cultures Europe ever saw... this thread is weird as fukk.