Obviously nine out of ten English teachers are just incompetent. They claim to know which words are important for conversations and which are not and that's just plain bullshit. There is no "list of words you have to know" or vice versa.
Fortunately, my advanced course teacher was really great. He just talked to us, we read English books and watched English films, really great films like 2001: A Space Oddyssey, Dead Poet's Society or Pulp Fiction. He never gave us vocabularies, we just used dictionaries and fetched the words we needed. We wrote fictional stories in different styles of speech (eg. Business English, Old English, Modern English) and after a while we just wrote with our own styles. There was no problem writing slang language or bad words like "fuck" or "shit". We wrote short stories, novellas and so on and that was something that really helped us learning English. If I ever became an English teacher, that would be exactly the way I would teach. It was great.