First you will need a screwdriver, i preferred to use a knife (table knife so it wasn't sharpe) and prize the vent out start from the window side and work you way round gentlely pull from the lip inside the vent or the moving part but this could be more risky if you pull to hard.

i also went and removed the rear ash tray surround which done by removing the ash tray inside to reveal one bolt undo that and pull, and the surrounds of the door lock pins which is just again a prize with a screwdriver to lift it off.

now you should see 4 clips or openings around the out edge, about 1cm down from the top the ones you need are the lower set of 4 clips. like this:

put a screwdriver under them, little watch and jewllery screwdrivers are best try to make a gap between the surround that you are going to paint and the rest of vent, i put a screw driver in the gap as well as i found it keep locking itself back inplace.

you should be left with the surround and moving vents/vent controls moving freely from the bottom part that is hidden when in the car, now if you look at the underside you will see a black flap with a rubber lip all the way round it.
do you see the clip and a pole connecting the control to this black flap, you nee to release this. close the vent control, this should close this black flap, now hold the top part by the surround you want to paint and pull untill it unclip from the black flap. YOU CAN ONLY SEPERATE THE BLACK FLAP FROM THE TOP CONTROL AND SURROUND WHEN THE BLACK FLAP AND VENT IS CLOSED.