Are you talking about the vents that are in the front by the windshield? Then you might want to remove that panel - carefully - and get in there and clean things out.
The air filter shown, if it can be made to fit, would be a good idea, but all it does is prevent dust from getting into the evaporator.
The other thing that we have had to do at times to take sections of the heating/vent system apart and use a shop vac combined with compressed air to blow particles out of the system... time consuming, but works. Some detail shops have machines to clean out air vents as well, might be worth a call to see who can do this locally for you.