Vinland Saga is really only seinen on a technicality. It was originally in a shonen magazine, and it only got moved to another mag because the author couldn't keep a weekly schedule.
Personally, I don't see why everyone has a stick in their ass about the distinction. The only reason anyone cares about shonen vs. seinen is because they're insecure and want to legitimize liking a kids' comic. Of course, the implication of saying "nuh uh, it's seinen" is that liking a shonen manga is something you have to apologize for.
And before anyone says "but shonen = naruto and pokeymanz," remember, Fist of the North Star and Jojo's Bizarre Adventure are shonen.