There are events that create specific regional cultures, and events that convert Barbaroi/Skraeling culture provinces to one of the more "acceptable" or assimilated cultures eventually if they aren't controlled by a native council estate.

The Varangian culture is only for the Varangian Order to my knowledge, the provinces will automatically reconvert when you move the Varangians back to the Old World. I think if the Vinlanders take territory that is Varangian culture, it stays Varangian, but the Norse group version of Varangian.