Thank you romantique. I eventually saw that Halion Sonic au units were visible in the Preferences/Plug ins pane of VEPro.
Then in VEPro I selected au plugin dropdown menu, instead of vst dropdown menu, and saw not only HS but also HSO as options. I was able to open both GUIs in VEPro.
Unfortunately both are stereo only, but this is great progress.
Since I do not use synth sounds, HS is probably of little benefit to me. But I am pleased to have now found HSO vstsoundsets available for download, and also an updater for HSO aus on Mac. It will be interesting to see if I find any differences or additions. (I bought HSO in November 2014).
Maybe I never had, or never installed, vstsounds for HSO.