How to vassalize succesfully?

[Raijin]

Recently I have tried as Seljuks. UHV seems pretty easy since your cavalry is out of the world. I can get the %World very fast with no problems but I just can't get any vassals. If I fight against them they collaps before I get any chance to vasallize them. They just don't want to be my vassals.
For example Zengid who I brought down to Mosul with only 1 defending unit left and still they refuse to get vassilized.
What am I doing wrong?

 

[embryodead]

One of the common reasons why the AI won't surrender even though it looks like it should, is multiple wars. So if Zengids are at war with another player as well, they cannot surrender to you.

 

[Raijin]

Which parameters are important to get vassals? Land and Army size I suppose but does stability effect this?

 

[embryodead]

As far as I know it's war success (i.e. if the enemy lost/gained any cities) and power (army size and to a lesser extent military buildings). Stability is certainly irrelevant.