Peter Harris
Warlord
Usually capitulated vassals, but on occasion both. Basically, in a good few months of solid bts play and many many vassals along the way, I have not ever lost ONE! I am quite sure that on some occasions they have met the criteria, but they never make the break.
One factor COULD be that I usually liberate most (if not all, if I don't need them) their cities back to them, and while this often takes them from furious, it doesn't always make them exactly love me - this also of course will often give them (or set them well on the way to) the pop or land mass criteria to break.
I dunno, not complaining like, I just wouldn't mind one break so I know that all is working properly.
Maybe the AI just love me (or know what's good for them) and they realize I'm a merciful and wise master![]()
I understand that people have looked at the code and it has been established that a vassal has the "OPTION" to break free if it has 50% of the masters land area and 50% of the masters military strength. That does not mean that it will choose to break free. The exact circumstances of when a vassal will choose to break free have not been definitively been set out anywhere that I can see.
The following is based on my own in game experiences with voluntary vassals (I don't take capitulated vassals):
- relations appear to factor into the decision (if the vassal loves you less likely to break away)
- whether the vassal is scared of smebody seems to be a factor (umh, you might be the scary monster

- trade may be a factor, not at all sure about this yet. I don't demand tribute (unless my vassal has the only one of any military resource such as aluminium) but I prefer to trade with my vassals and I gift them ANYTHING I can spare.
- the vassals chances of winning by peaceful means may be a factor, not at all sure of this yet. An advanced vassal with some juicy cities has some chance of a space race win but no real chance of a conquest win if it broke free.
- the aggression level of the civ may be a factor, not at all sure about this yet (I always suck up to the small peaceful civs to lure them into asking to be my vassal and I don't lose them but I have lost nasty ones such as Brennus)
Edit: Neversurrrender, I guess you must be a merciful and wise master!