I have a relation of 200 and I've had the vassal for more than 10 years, but when I press Demand Annexation it says Impossible... weird.
Politik är aldrig fullständig; den måste ständigt utvecklas
- Göran Persson
You can try to annex them. You just have to improve your BB, prestige and hope you have a good leader and they also have a good one to improve your chances to get them to accept a diplo-annex.
For some reason you can't annex a vassel that is of a different religion then you. Well thats the problem I am having as the Ottomans trying to diplo-annex my christian vassals :(
How many GMs does it take to change a burntout lightbulb?
None, it is working as intended.
Having a low reputation, high prestige and a county leader with high diplomatic skills help.
In In Nomine, you have some important person that also increase your dimplomatic skills.
If you have difficulties to sign treaties with other contries, be sure to have all above things first.