Actually, I've heard of that sort of thing happening a lot, though it doesn't often happen to me.

Generally, if the AI approaches *you* with a Ceasefire offer, you have them right where you want them, and can get all sorts of goodies (like vassalhood) out of them.

I've heard of Vassals (and Protectorates in RTW) being much easier to get under those circumstances you just described: AI attacks you, you beat them in the field very shortly thereafter, and then they desperately want peace.

The thinking was that the AI faction will have better relations with you after a very short war, whereas if you beat them down to their last city, you'll have made them so angry they'll never consider becoming your Vassal.

This makes me wonder if there was an invisible "Global reputation" score in RTW as well...