As to your speculation that changing your SSD from APFS to HFS+, how did you make the conversion without erasing/reformatting the SSD and re-installing High Sierra?
That's exactly how I did it: installed High Sierra, set up all my apps, cloned the lot to another drive. Then, erased/reformatted and cloned the new system back to the SSD.
In either case your best bet may be to re-install MacOS from the Recovery Drive. When running a re-install from the Recovery Drive the key combination you use to boot the Recovery Drive is critical.
With this method (Option+Command+R), do you lose your User data? I presume I would not be offered Mojave, as I do not have a Metal-compatible graphics card.
Since posting, I have run OnyX. The VLC system freezing problem did not reoccur, so now this just becomes idle curiosity.
Thanks, though, for your help!