In a couple of different playthroughs I had a situation where vassal annexation stopped.
In the first case it was a single province territory (I played by France, had 100 provinces or so and wanted to vassalize I believe Frankfurt) - I could start vassalization (I had relationships higher than 190), but was not able to have more than 0% progress.
In another case (playing as Crimea, I had around 15 provinces and wanted to vassalize Georgia, they had 3 provinces) I started vassalization, earned around 50% ... and during this time completed the annexation of another vassal. As a result my relationship with Georgia became worse (dropped to 120 or so) and vassalization stopped.
I started improving relationships, pulled them back up to 160, but that didn't help to resume vassalization.
Any ideas why?