I have always put them next to the rear view mirror on the windscreen. They are so tiny I can't imagine they could be seen by a camera in any detail? I mind you it would be able to tell whether you had one or not but I suspect not whether it was in date.
The law does not stipulate the precise place where the vignette should be affixed on the windscreen. The valid vignette can be placed anywhere on the windscreen (not in tinted strips) as long as it is duly properly affixed. The illustration on the back of the vignette is a recommendation by ASFINAG.