"Veganism" is an ideology, a product of the same capitalist mechanisms that created the global meat industry and turnrd meat and other animal products into a daily staple for much of the world. It's a reaction to that reality; a moral rejection of animal product consumption made possible by the fact that now we live in a world where you can feasibly only consume plants year-round.

The vikings couldn't be vegan, not just because their environment probably necessitated them eating some animal products, but because the very concept of veganism wouldn't be able to emerge in their world. The proper conditions didn't exist yet.