Downstairs easy, just do as above and then give them a wipe with scrim as you would've done before.
As for the upstairs..Against all the good advice, I decided first time around to try and give them a good old wash around and warn the customer that due to me cleaning the vent more thoroughly than ever before, ie getting a rinse inside a bit to get the black look gone, those windows may drip a bit from there and leave a few spots, but from then on, leave the vent alone and follow previous suggestions.
They stay clean quite a while if you are prepared for the first bad result, and I have found as long as the customer knows what is going on they are happy. In fact pleased they're getting a bit extra.
I have found it best that way, the vent is relatively clean inside next time round so you don't have to be quite so cautious round it.