Do I need a vapor barrier on an 8" pour + carpeting over tile?
You don't mention what your climate is or whether you've had moisture in the basement. My answer is therefore in general.

Vapor barriers are rarely "barriers", and generally act as retarders. In this case, your tile is a fairly effective retarder. I'd discuss it with the carpet supplier/installer. It's often the case that carpet pad has a built-in moisture barrier as well.