One of the best joke I saw about that shit was like "Hey, you know how those ancient aliens only seem to help building Egyptian pyramids, pre columbian and Asian temples, Moai, etc...? Only logical conclusion : looks like aliens must really hate white people."
Among the ...creative "historians" who plague French-speaking youtube, there's an idiot who spends hours and hours claiming all of the cathedrals were made by vikings to spread the true faith, fucking Odinism, against Christianity.
The guy doesn't even know what a "cathedral" is (literally just a church with a bishop in it), and cherry picks examples (some of them not cathedrals) that he can make roughly make correspond timewise (ignoring dozens of cathedrals long before or after the relatively short viking expansion).
He also quotes nazi pseudo-historians, theories about the templars being vikings and establishing South-American empires, and he presents himself to his most devoted cult members fans as the incarnation of a Norse deity. He hasn't made up his mind about whether he's Heimdall or Hermod though.
In exchange for land and a title, they converted as a formality and where indistinguishable from the rest of French nobility in a generation or two. Basically an afterthought.
Contrary to popular depictions, vikings were generally pragmatic people.