The title says it all
#Is vassalisation a diplomatic option in AoW 4?
fluid nimbus
fresh zenith
Yes.
It's easier and more beneficial for some builds than others.
We don't know all the details just yet.
shy silo
Do you mean taking city states as vassals, or defeated rulers as vassals?
tame marsh
Did you even watch the reveal stream? It was talked about that you can. Unless you mean something more specific