Unfortunately they are not functional and there only for aesthetics, as you can see on this pic:
The vent cover is plastic, held in place by clips. It can be pulled of with minimal disassembly by unscrewing/unclipping inner guard on the bottom, and unclipping the side skirt in the vent area.
All of these could be done with just dremel, but making it clean would be difficult.
Not pictured: inner side of plastic vent cover has a rubber flap, I removed it as well.
In the end I am happy with this little mod, even by having radiator fan on on stationary car is enough to feel warm air coming from the vent.