Which explains my point (far more succinctly than I can) that CK2 "feudal" mechanics don't really suit the Northern Lords.
To bear in mind with how Winterfell views its vassals however- the Stark Lords give the chiefs of the Mountain Clans the style and treatment of Lord, just as much as the Karstarks, Boltons, Umbars, Manderlys et al. Therefore, it may be clever diplomacy on Winterfell's part, rather than the actual "de jure" status, and clearly isn't the de facto set up- I believe that even combined, the Clans aren't able to raise more men then Manderly? Although I could be wrong with that last point.