I've been using "Vision" for a couple of months now and have found that on windows where I've always had problems with spotting then the spotting has been either removed completely or vastly improved.
White upvc frames do not go dull as a result of being cleaned with WFP, that's down almost entirely to oxidisation from sunlight, age of the frames and location also has a bearing of course.
If your customer has bought top end frames then some of them have a plastic coating over top of the upvc and they do not oxidise or go powedery at all.
Much like the brown frames with an artificial hardwood veneer on them, UV rays will dull them over time but they are almost always good to clean.
The sudsing effect that makes bubbles on the glass is a little disconcerting at first but I'm assuming that is down to Vision lowering the surface tension of your water and by doing so will also make your water 'wetter' is, it will penetrate soiling quicker thereby meaning it should help make Wfp slightly more efficient at cleaning windows.
It may only be a marginal thing but for me it will always be part of what I use to clean windows.
I also very rarily rinse off the glass and have found no need to do any more rinsing than normal.
I do plenty of inside and out work to know that my own technique works just fine.
Its good stuff in my opinion
