Set War Goal Vassalize (Subjugation)

[Jor]

Hello,

how will i get this War Goal?

I Propose Subjugation, he refused.

I can deklare War, but only to Conquer or Humilate.

His Fleet is inferior, His Technology Level is Superior, His Economy is Equivalent.

Is Vassalize only possible if I am stronger in this three Parts?

[Sero]

It only says you need to be superior to them in power.
you got one equal and the other two seem to balance each other.