Vassals were truly the "hidden" Conquest/Domination VC accelerator. That was the true intention of the mechanic, and it succeeded at that. No "capture all capitals and ONLY the capitals" nonsense. Emerging nations was not part of civ 4, but became a fun part of one of the (many) best mods: Legends of Revolution. If you leave barbarian cities alone or unconquered long enough, they would emerge as a new civ after some time. Amazing! Also, civs could split into two when revolutions hit, due to many factors that made your pops unhappy.