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
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.