I'm currently in the mid-game of Stellaris and building up a pretty big empire. From beginning on I had three neighboring, strong empires. I've beaten two of them recently, and forced one to be my tributary and the other one my vassal.
Now I would like to conquer my third enemy…but in the war conditions menu I cannot choose to vassalize him. I can only make a tributary of him. Why is that?
I've also checked some other (small & big empires) and it seems that I cannot vassalize any of them. I can only choose the tributary thing in the war conditions menu. Can I only have one vassal at a time? I've integrating my vassal at the moment: does this affect the vassalization of other empires?
Best Answer
You can vassalize by liberating a group of systems, then vassalizing the resulting empire formed of those systems. The new empire that's created as a result of Liberation has a large bonus to their opinion towards you because of the Liberation, so you should be able to vassalize immediately.
If you have a Defensive Pact or higher (Alliance, Federation) with another Empire, or they are a Protectorate, you can't vassalise that Empire.