When demanding vassalization, a number of factors determine whether the other empire will accept your demand:
One of the biggest factors is "Distance". What does this distance signify?
It does not appear to be distance to my closest planet: in the screenshot bellow the only planet of this empire is in the same system as one of my planets (since I just liberated them, and I took over another planet in the same system in this war)
I don't think it is distance to my capital either, the system is fairly close.
As per this question How to get other empires to agree to "demand vassalization" peacefully? liberating planets should make it easy to peacefully vassalize them, yet this almost never seems to work in my case.
Here I reloaded and liberated a different planet in the same system. The distance stat appears to be gone. (Still not enough to demand vassalization unfortunatelly)