I'm not sure I understand what you are asking, so you may have to correct me. You are asking about modeling the parts of the vent you cannot see in the reference image, yes? The only option in that case is to just make your best guess. If you have photos of the car, I recommend looking at those to help.
In the end, do whatever looks good and will fit with the surrounding parts of the car. It will take practice to get good at those parts of the car, so don't worry too much if it takes a while to get it right.
And sorry for the late reply! Usually I am much better about that.