I would say It has nothing really to do with if vents are open or not but is just down to your wfp technique if the glass dries out with spotting/drips marks. With a bit of practice I'm sure you will figure out the best way and get the hang of cleaning problematic windows with vents.Worth mentioning hydrophobic glass can be harder to get good constant results than hydrophilic glass. But again that's down to wfp technique used.