To liberate a city of a vassal state

[frktest]

I declared war to a AI player and thus to a his vassal state as well; the purpose was to take the capital of the vassal state (that was conquered by the AI player) and then to liberate it, but, as I can see, that is not allowed by the rules. I think that it should be.

 

[Bhawb]

You can't liberate cities to civs you are at war with, for fairly obvious reasons. This applies to vassals, since you cannot war the master without the vassal.