Viking raiders are pests! It's easy enough to beat them or scare them off, but can I eliminate the problem by converting them? Will Catholic Vikings continue raiding other Catholics? Or do I have to seek other ways to eliminate their raiding?
Only pagans are allowed to raid. Converting them would solve the issue.
The question would be how you plan on converting them. Sending your court chaplain to try to convert them is generally just a good way to get him thrown in a cold dungeon somewhere in Scandinavia. The only way to do it efficiently would be to seize control of the territory yourself, and then the problem goes away anyways.
All in all, it's probably more trouble than it's worth. The AI generally sends groups of raiders too small to be much of a threat or to do much economic damage. They are pesky, but I'd say your time is better spent on other things.
another solution: Coast guard - take a look http://s13.postimg.org/q1700qpad/coast_guard.png