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.