one of my little tricks is about getting a vasall. I've done it like 8 times now (in rtw, too) and it still works.
you corner an enemy faction to have just _one_ city left. then, you send your diplomat to that city, and ask for their vasallage. they'll most likely decline. now, you send a huge army in front of their town, and if they have a unit or diplomat around, you can even lay siege to their last hometown - but don't attack. then, you negotiate again - either to the city or if you're laying siege run up to the enemy unit or diplomat or whatever and ask them again. with this tactic, they've ALWAYS accepted for me so far. they "barely accept" but yea they accept. you automatically take siege off the city if you were doing so.
that way I've gotten selucid in rtw, england + scotland + egypt in m2tw and so on. it pays. you get all their money