Cut, Burn, & Cover:
FRAME SHIFT: As fanatical as we Minnesotans might be about our Vikings, horns would be a serious impediment to normal human behavior. Actual Vikings did NOT wear horned helmets, because they were impractical and would provide leverage in combat, so a blow against the very broad horns might twist or break a warrior's neck. If those horns were actually IN the warrior's head, it could be quite lethal. Not to mention sleeping difficulties and neck/back pain. It's likely the presence of Viking-helmet-style horns might adversely affect brain development. Not to mention that I am just a little insulted on behalf of my Norwegian neighbors - I can't picture Knut and Signe with horns.
Such horns would mark Scandinavians out as being non-human, and the Vikings needed to maintain a quite large trade network where their reputations were important. While Vikings MIGHT have liked terrifying their enemies, they knew the advantage of getting along with people as well.
So it's likely the Vikings would trim and saw their horns and burn the stubs to remove impediments to their social and military effectiveness. Then they would likely take up wearing lots of hats to cover the stubs. "Horns? What horns? That's a Thor-cursed lie to make us look bad. What, these bumps? From a childhood religious ritual. You aren't on the inside, so I really can't share the details..."
Such horns would, however, provide spots where the helmet could be made snug to the head. For those die-hard cases where they refused to "fit in," they would likely need to go without traditional helmets (these would be more akin to berserker-type troops: there more for terror than practicality). If they had helmets they would need to protect their horns from injury, or partly trim them back to make them less vulnerable. In the same way spear shafts had an iron guard along them, the horns would need a guard or wrap to shield them. Perhaps a two-piece clamshell helmet that slipped over each horn, then pinned together to a middle plate. Otherwise a very minimal helmet over the top of the head (and yes, likely slitted to go over the horns) would be the only practical solution.