I think that while it maybe possible for those who acquire a massive vocabulary to become capable in expressing whatever that they want to say in any given situation, if they do not have the proper listening comprehension skills to go with their other skills such as their essay writing or conversational skills, they could not say that they have possessed skills that are at par or superior to the natives. I think one can claim he/she has acquired native skills when he/she can do anything that the natives can do, or more.
I didn't expect to discuss English here
but from what I know, in this case while the word "proficient" maybe a more proper word for some individuals, I think that "fluent" can be used here as well. Below is taken from Yahoo! dict.
fluent: 〈言葉・文体が〉流暢(りゅうちょう)な, よどみない;〈人が〉(言葉に)堪能な((in ...));すらすら書ける, 達筆な;口のうまい
proficient: [形](…に)上達[熟練]した, (…が)うまい((in, at ...)). ⇒SKILLED[類語]
In 〈人が〉(言葉に)堪能な((in ...)), The word 堪能 has a meaning of "深くその道に通じていること。また、そのような人や、そのさま."
In English, 堪能 can be translated as "good ((at)); skillful [skilled] ((in)); proficient ((in))." (goo.ne.jp)
〈人が〉(言葉に)堪能な((in ...)) may then be translated as "a person who is proficient in languages", using the definitions above.
I apologize for digressing.