I've had two instances where I won a war and enforced a peace, then was forced to load an earlier save for various reasons and no longer had enough war support to enforce the same peace on the same turn with no other things changing between instances. I do have two save files which can show this behavior (one where I enforced the peace and the autosave from the same turn before I enforced it), but unfortunately one of them is broken and doesn't load (which is why I had to reload the autosave to begin with). I've included them below. In the "Pass the Dutchy" save, I've enforced a peace which included all the required demands + vassalization of the opponent. In the autosave, I no longer have enough support to demand vassalization.

Pass the Dutchy.ctrauto-save 751.ctr