its probably best to hold them personaly, because the jomvikings countys tend to be feudal, if not from the start they do real fast, and if you are still tribal when you hand them out the vassal usualy ends up tribal with wrong holding type, and will proceed to build tribal holdings in the empty slots, which blocks the jomvikings.
if you cant hold them personaly, then contriving a way to make sure a feudal vassal gets them and doesnt fill the empty slots is best, but i do not know a surefire way to do this.