We don't really know. The Vikings left precious little behind in terms of literary evidence, as most of what we have that refers to Cosmology (the Prose Edda, for example,) were in fact written in the 13th century, and by christians at that. According to the Prose Edda, the world is not spherical, as there are several worlds which are usually interpreted to be flat with a domed sky (the Skull of Ymir.) but again, it's impossible to be 100% certain what the Vikings in the 900s would have thought because it's not clear what elements of Norse culture were changed when they were written down after the fact.

That's a great question, but it's probably best to begin with the assumption that people in Viking Age Scandinavia wouldn't likely have been familiar with the Greek idea of the cosmos. I'm also not sure they'd be all that interested. Their artwork and surviving literature focuses much more on human relationships and animal life (although some of their gripping beasts are quite imaginative) rather than the cosmos and the world.

A question recently came up where I got into a lot of these same questions from the perspective of Did the Vikings think the earth was flat? My short answer was probably not—although they didn't likely think of the world as round either. To cut out the, erm, elaborate discussion and jump right to my own conclusion (with some edits for typos):

In short, most people in the Viking Age would have experienced the world as a network of routes, and there's no evidence that they ever paused to imagine what these routes would look like from space. Many of them had the opportunity to make the observation that the earth is curved, but with the vast expanses of the North Atlantic on one hand and the endless Asian steppe on the other, there was little reason to think about where the world ended. Some might point out that there is a myth about the Midgard serpent that surrounded the world, but, again, this 'world' should be more properly thought of as a homestead in a larger world rather than a self-contained planet. The myths that do survive similarly suggest that Viking Age peoples conceived of these homelands scattered across a cosmography unified by a tree but had no interest in where this tree's world ended. Most importantly, because these people seem to have thought about the world mostly in terms of journeys from point A to point B, and because they didn't value perfect shapes and surfaces like the ancient Greek philosophers, there's little reason to think that they would have moved from the observation that the world is curved to questioning much less assuming that the world might be round.

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