I'm not sure if this has been addressed before, but I feel the ability to liberate vassals should become immediately available when there are no active civilizations left. I was doing a domination victory with the Aztecs, and everything was going great. I had a war on two fronts with the last two civilizations in the game, and I was only able to take out Japan earlier than anticipated, because Sweden agreed to capitulation. Once taken care of, I thought to myself, "This is perfect. I used a citadel to take the territory around Stockholm's borders. All I have to do now, is move in my cannons, liberate them, and finish them off." Then I realized I had to wait another 45 turns before I could liberate them. Now, I'm certain I'm not going to finish a clearly finishable game, because I don't want to spend the next 45 turns doing nothing of value just to get a winscreen.