I guess it would be cautious to say that this is just my own little theory about how to come to master a language and in which vocabulary levels a language is split up. After this little disclaimer, the theory: I think that the mastery of a language is the better the more and divers vocabulary is actively and passively used. Kind of obvious, hein?
Well, the challenge of really having the command is to be able to use the language in any given situation: playing football, going for groceries, reading a newspaper, going out for a beer, writing a PhD thesis, etc. Personally, I have the feeling that I’ll never finish learning French or English, especially in the extreme levels (insults, familiar language, technical terms) though it is true that even native speakers do not know every single word.
The point is that to learn a language, it is not enough to study and to read newspapers, you have to go out, experience, get lost, make friends, learn from them as much as you can withouth necessarily adopting all four letter and similar terms they may teach you and live with the knowledge that you will never make it to 100%.
Après tout, c’est pas si compliqué.