Well, that's no problem at all if you are the playing the Vikings. If you are talking about the AI, then yes, the Vikings tend to run out of steam after a while. I assume its because they have no cash and the AI for some reason makes bankrupt factions passive. If I had time and inclination, I'd mod the game so that there are two Viking era campaigns to pick from. In one the Vikings are unplayable and the Viking huscarls are much cheaper, so the AI will keep the Vikings going. In the other, all factions except the Vikings are unplayable, and Vikings have to settle for a weaker royal units - maybe nerfed huscarls or something. Doubtless other tweaks would be needed to improve AI Vikings and weaken player controlled Vikings.