Fully open the vent and then from the outside if you look under the vent you will see two bolts holding it on. Remove these bolts. Then, the only things holding each vent to the body are two split pins. Pinch the open ends together with pliers, then carefully tap them out with a flat screwdriver under the pins head.
Its easy to slip and scratch the bulkhead with the screwdriver, so wrap some tape around the end of the screwdriver to protect the paintwork. Or use a piece of wood instread.
The vent knobs remain in place. If you ever need to remove them (as I did last week) you need to partially remove some of the dashboard. The vent knobs ara also held on with two bolts. Not a difficult job, but getting the grey dashboard part in and out can be tricky.