Most of these vents can be 'cleaned' with a wet brush and no water flow. Just get the brush bristles into the vents and agitate the dirt loose. Clean your brush lower down on the window with the water flow on, switch it off and go back to the vents and brush them again. You may have to do this several times.
We have found that with most vents, rinsing above the vent will help to rinse it out with water ingress, so use the opportunity to clean the frame the vent sits in at this time. Again switch the water flow off and brush the excess water off the vents.
You will have to come back to clean the windows later, this time just the top edge of the glass and then everything else below it.
It takes time, so change extra for the service.
There are some vents on our round that can't be cleaned that way that I would stay away from; we are window cleaners not ventilation specialists. The ones I am referring to are recessed horizontally into the upper window frame and reply on a drip plate trim above to prevent water ingress. But the drip plate isn't for water fed pole jets spraying water horizontally.