If a AI has another AI as a vassal and i pass some newly conquered cities which i don't need to the vassal so it can grow a bit larger will the vassal AI break free? same question about my own vassal if i decide that i wanna kill him.

In theory, if you pass the vassal enough cities that he breaks the percentage threshold, he will have the option to break free, but it's not clear that he necessarily will choose to do so. Try it and find out.

Another determining factor for the vassal is his apparent power status. Hence, you might want to gift him so units too so that he feels big and bad.