Good question! Don't think i've got those on mine although i have the outlets. I know on the Rover door cards that have something similar, when you look on the other side of the door card, there are two star washers, one each end. Pry them off using a pair of external circlip pliers underneath them, use a flat punch to reshape them for refitting then pull the vents out.
Not sure if Volvo have used a similar technique but wouldn't be surprised.
On another note, i keep thinking about adding some walnut veneer to the door card inserts, the ones obviously missing from your doorcards in the pic. Currently got a dark grey fluffy material on mine and i miss seeing a nice bit of walnut veneer in the car!