If you have the option to annex, then you have filled all the requirements. But nothing in this game is certain - there is still a chance they can refuse, and that is affected by monarch stats (yours and theirs, I believe), the length of time they have been your vassal, and the relative strengths of your countries.
In most cases, it is to your advantage to wait until you have an extremely good monarch, unless the vassal's provinces are of your culture (Brittany's are not) and religion (which it is), in which case the economic benefits are in favor of annexing as soon as can be managed.