"Are you outside?"
Translation:Bạn có đang ở ngoài không?
According to the advanced learner I have access to, the "có" is always required in yes/no questions to be grammatically correct. However, it's perfectly understandable and often more natural in relaxed speech to omit it.
He said to compare it to the English do/did. It's correct to say "Did you eat yet?" But often people simply ask "You eat yet?"