Vikings came from Germanic tribes and Ice Age people in Europe.
Celtic women before them had all these rights and became of a clan or a tribe example to Queen boadicea.
Vikings did practice polygamy. It's correct that there usually were just one wife, but there was also a concubine system, frillas, these were offical relationship that gave the women and her children certain rights. Frillas came for lower status families relative to the man so they weren't a social threat to the wife whose status rested on her fathers family. But of course these things are all about what resources a man commanded. Most men wouldn't be able to afford to support one woman. As Adam of Bremen describes the Swedes in the 9th century. "Only in their sexual relations with women do they known o bounds; a man according to his means has two or three or more wives at one time, rich men and princes an unlimited number. And they also consider the sons born of such unions legitimate. But if a man known another man’s wife , or by violence ravishes a virgin or spoils another of his goods or does him an injury, capital punishment is inflicted on him."