I have found the easiest way to clean these is take them off and lay them in your sink and run hot water over them for a few minutes. I liberally sprinkle baking soda all over the vent making sure to get it into all the little nooks and crannies. I then spray the entire thing with vinegar. This creates a chemical reaction that should fizzle a little. I let that soak in for a little while, then I use a stiff brush to clean it out. If you have an old toothbrush laying around that works well too. I usually spray it one more time with vinegar just to make sure it is grease free. Rinse it with hot water and viola you should have a clean vent.
The more you cook the more often you need to clean this. I suggest cleaning it at least once a month. If you do, that task won't be as difficult the next time!
Do you have a question for The Cleaning Fairy? Let me know in the comments!