We do get reports of complaints on here from time to time. Not a lot, but once in a while. I also read about people who have quit vaping because they think their health problems are caused from vaping. Our little group, usually, quickly tells them that it's likely residual effects from previous smoking. This may very well be the case, but we all do have a vested interest in believing in the relative safety of vaping. So you're right, we don't hear too much, but when we do, we tend to want to blame something else.
I do pay attention to those threads when I see them. I try not to jump to "it can't be vaping, it must be ____" though it probably does happen. The problem is, most things can be attributed to something else, like quitting smoking, or the particulars of how the person is vaping, or even just their own medical history/physicality.
I'm not saying that no one will have an adverse reaction to vaping, I'm saying that on a large scale, there hasn't been anything that we can definitively attribute to vaping.
As for diacetyl and other potentially hazardous flavorings, those concerns came from the community, have been explored by scientists that the community believes are fair to vaping, and even then it hasn't surpassed "concern" level, though some have taken it well beyond that.