I don't know how much they were actually used by pilots in WWII, but even modern aircraft continue to have them.
The F-15 had such good visibility, it was built without mirrors. The first thing Israeli pilots did when the got them was add mirrors. The US pilots quickly made sure they got the same modification.
I don't think vibration would be much of a problem for the mirrors faired into the canopy frame. Any vibration that would have upset a mirror image would have affected the gunsight as well since both were rigidly connected. The ones on a narrow stem with no streamlining would be more likely to oscillate.
If the mirrors were useless, pilots in the field would have removed them to clear their field of view and reduce drag. So, I suspect at least some, if not the majority, of pilots probably used them to some extent.