In addition to giving the Viking provinces merchants (as Wes has done), which in my opinion is a very good idea for the AI, there is another complementary modification which i have found to improve the terror factor and lastability of the AI Vikings.
Simply give them ~2 extra ships at the start.
One of the most common ways of the AI Vikings becoming ineffectual is by them losing enough ships to prevent them from raiding. They have 8 starting ships and it can sometimes be a considerable length of time before either of the Viking provinces are capable of building boats.
The AI will always position 1 ship adjacent to each of their home provinces and one ship in the middle sea separating Britain from their homelands. The extra ships allow them a little more variety in their raiding options (something the human player would instinctively have with only the 8 starting ships) and will also mean the Vikings will benefit more from trade at the same time. That is, assuming they are not at war will all the other factions.
EDIT: corrected the number of starting ships lol http://www.totalwar.org/forum/non-cg...cons/smile.gif
