During several playthroughs I found out that AI declares war on you only if you are about ~10th in a global strength rating or lower. If you play as a strong faction from the beginning (like East Anglia or Wessex), AI will NEVER declare war on you. On Hard and Very Hard at least, I don't know how it is on legendary.
Which is kind of makes the game broken. You can chill for 100 turns and never get attacked.
I don't ask to make AI suicidally stupid and declare war on you just because why not, but for example when you play as East Anglia, and have -100 relationship with Wessex and they have 6-8 full stacks near your borders... I think they should attack. But they never do, they rather go to war with some scottish guys in the far north.
And some concerns about vassals - you still can't vassalise factions on your own, only with scripted events. You can get a vassal only if some event pops up, or you conquer the enemy faction by capturing their last settlement. When I try to vassalise some minor faction with 1 tiny settlement with which I have great relationship and I basically the only one they are on border with, this faction will NEVER accept to be my vassal even for ridiculous amounts of money. Like 3000000. It's always insulting for them. You can only declare war on them, step on their last last settlement with your general and only then they will become your vassal. Which is not really that logical at all.
tl;dr AI too passive, making vassals is impossible. Please patch it guys.