Negligent discharges CAN happen while decocking, but the two negligent discharges I've been guilty of had nothing to do with decocking. (Both were with guns with magazines removed that were empty.. Or so I thought.) The first one was many year ago, and I thought I had learned a lesson. The other one was a few years ago. Same issue. No excuse. Just dumb. But the gun was pointed in a safe direction and there was nobody nearby who could have been hurt.
Have you had a ND with your Vaqueros? If you had one with your semi-auto, you obviously didn't have a finger between the hammer and slide when you decocked, or didn't hold the hammer with the other hand as it was lowered. The fact that it was pointed in a safe direction removed the risk of harm to anyone else . If you have a ND while decocking a weapon, it can be an embarrassing event, but that's about the worst thing that will happen.
Fear of somebody being hurt seems to be the main reason so many individuals are wary of manually decocking a handgun. But many of them are new to handguns and are also uncomfortable handling a loaded weapon. I guess that's probably good.
The next time you go to an indoor range, check the walls and ceilings. You'll seen many more signs of negligent discharges there than you'll see when looking at the floors. I don't think that most of those marks and holes were decocking accidents. I think they were people doing things they ought not be doing (like practicing presentations, if it is allowed at the range.). The stuff that led to those many bullet holes or marks were probably far more dangerous than a decocking accident, but nobody ever mentions that sort of negligent discharge.
I haven't shot IDPA for a number of years, but for 5-6 years I was one of the safety officers at our local club matches. Except for the guys shooting Glocks, most of the guns had to be decocked before starting a new string. A lot of 1911s, and few CZs (a few with safeties and one or two with decockers), and a few SIGs that had decockers and a couple of revolvers..
Never observed a negligent discharge while someone was decocking in all of those IDPA matches over the years. Could it happen? Yes. But I'm pretty sure that nobody'll be hurt.