I always find it entertaining when people say "fighting honourably" when they mean "fighting stupidly" at least from a tactical point-of-view. The Brits/English often accused their enemies (the Zulu for example) of doing this.
... so? Who won the fight? Okay... shaddup then.
Yes, there are sometimes recognized rules (such as Geneva Conventions) for warmaking but most of the time... whatever the victor did in order to win eventually becomes considered legitimately acceptable.
As for the original topic... it's the age old "naval superpower" versus "land/army superpower". Sure, the Vikings weren't united as a naval superpower, but they didn't need to be. And all throughout history, it's the NAVAL superpower who usually won, from the Greeks thru the Vikings on to English and up to America today. Sure, there were exceptions, but generally speaking... if it's a land superpower vs a naval superpower... the land superpower is likely to lose.
And that means that I doubt Charlemagne would have beaten or prevented the Vikings.