Is it possible to have a concept without the vocabulary to describe it? I would think so because mankind invents stuff and then names it. And then there's the whole allegory, science fiction, apocalyptic, seeing into the future stuff.
That said, having a speaking vocabulary but not a reading vocabulary is a major problem which requires more thought if dyslexic kids hope to meet their academic potential. I used to think that what was important was having the story line that shapes culture. Renting videos of Shakespearian plays, Dicken's novels, the Bible would keep a dyslexic from falling behind in the area of great ideas. Now I'm thinking that watching the movie version isn't good enough because the movie version doesn't build vocabulary. Life is too short for me to crack open Dickens. I challenge you to pull a Shakespearian play off the book shelf as a test of how many words are no longer in common use. Does it matter? Can you read across academic fields, across cultures? Can you do your own research, ask the right questions, formulate ideas outside your box? If your children choose to only have a speaking vocabulary, is that ok or is that to the brain what choosing not to get off the couch is to the body?