Taking a Vassal Faction? How?

[Midnight]

So, if it costs you (lots of) florins and territory, and they can just back out whenever, what's the point? Why should you go this way rather than just crushing your enemies and taking their lands?

I never used it in RTW, and I've yet to use it in MTW.

[Haudegen]

The best reason in my opinion is that the vassal can serve as a buffer to other potentially hostile factions that don´t border your empire yet. Additionally the vassal gives you all his surplus money every turn as tribute. But usually it isn´t much if any.

[Jacque Schtrapp]

The best reason in my opinion is that the vassal can serve as a buffer to other potentially hostile factions that don´t border your empire yet.

Only problem with that is your vassal is probably allied with the potentially hostile faction which will cross your vassal's land and attack you anyway thereby causing your "vassal" to side with his old ally and drop his vassalage on the spot.