@Northern Norse cosmology during the Pagan-Christian transition period clearly said the world was a disc, so the Vikings did believe that the earth was flat. When people talk about the Flat Earth Myth today, one important detail tends to be forgotten. That is, educated people - those who understood that the earth was not flat - accounted for less than 1% of the population. The vast majority of people believed whatever they're told by their leaders and elders. Said leaders and elders were more than likely to be uneducated or, if educated, not necessarily in natural philosophy. These leaders would very likely just resort back to cultural myths and traditions if the question was asked of them. If you're an Odin-worshipping uneducated Norseman, it's very likely you would have thought the earth was flat simply because you'd never met a monk educated in the classics in your entire life. When you met one, it's also more likely you'd be doing something nasty to him in the next 5 minutes. Education wasn't what a Viking would have been looking for in a monastery after all.
Now, sailors were a special subset of the population who probably came from an uneducated background before the 14th century. As modern-day people we are inclined to think that they would not be able to perform their trade without (what to us is) the basic understanding that the earth must be a sphere. However, this is an anachronistic way of thinking emerging from the way technical trades are taught to modern tradesmen. Tradesmen in the medieval world didn't need to understand why or how something works, just that it did. Sailors could function perfectly well in their trade without an understanding of how the world was shaped. Most of them would have lacked a conception of "the world" to begin with. We in fact know this to likely be the case from the extensive studies done on Polynesian sailors in the early 20th century. These sailors were perfectly capable of long-distance navigation in the Pacific, sometimes even crossing the ocean directly without doing any island-hopping. They were able to do stellar navigation and they had a concept of inertial navigation. However, when they were asked to describe the shape of the world, they would often just get confused by the question or simply return to their creation myths for a description. The sailors could see the curved horizon, but none of them bothered to even consider what it meant in terms of the shape of the world. The Vikings most likely also had this kind of attitude.