I find much to my joy after the diplomatically dubious AI of previous TWs that the AI has a sense of integrity in this one (I.e. won't stab you in the back on turn 3 of your alliance or continue a war after losing for 50 years). If I take time to cultivate a strong alliance, it usually survives the Realm Divide. Typically my vassals only turn me if their powerful enough function as notable clan independantly (I had one game when I played as the Oda where I made the Tokugawa my vassal and they went crazy with conquering every one they saw until they were almost as big as me, needless to say they turned on me with the Realm Divide) so I generally make other alliances to curb them.

Developing your own Matchlock units I find helps alot. Cheap, quick to make armies with a punch really level the playing field. It also takes awhile to research them so you get time to build up your economy.