Not sure what kind of vent you mean, but I had to clean my air conditioner the other day and I had half a bottle of Windolene. I filled the remainder of the bottle with tap-water and sprayed the slats and through to the innards (a/c was unplugged!!!) The continuous spray action moved the dirt right off the surfaces even those thin thin slats I couldn't quite get at with my fingers. I then used an old (clean) eyeshadow brush to wipe the areas I could see and then let it dry (couple of hours), plugged a/c back in and ran it for an evening and was pleased to find not only did it work better but there was a fresh clean scent emanating throughout the room.
If this is a vent like in your wall or floor, you might want to get one of those dusters that squeeze through slats like the static duster
![http://www.magent.com/wpeB.gif](http://www.magent.com/wpeB.gif)
which I've used to clean radiators etc. I've sprayed furniture polish on this duster and swiped it over ornaments and bookshelves and it does a great job.