Do left and right of the vent using loadsa water. Turn water off and dry brush the vent, get your bristles right up in there, poke it out, all the loose gunk and mucky stuff. Have a minute then wash the glass below as normal. Then, to save loads of time on subsequent clean's, keep clear off this part of the window till this time next year then repeat. If they complain, they are far far to fussy. Shoot them with a big gun and say....complain now ya dead bastird.