@fifonik, thanks for creating this thread. There seems to be a couple of issues being reported on the other thread under the "news" forum but probably unread by many of the users here.
Yes, I have the same feeling and I don't understand why this is not in this forum. Here is what I posted over there:
I just updated to the latest build of VP15 and may have stumbled across a bug.
I tried to selectively prerender an event using Magix AVC with VCE and that didn’t work. The project is a 4K project with XAVC-S files from my a6300. I can see the prerender is running but when finished I don’t get the bar across the selected section indicating the prerender and the playback is slow which means Vegas doesn’t read the prerendered file. It does however work with other codecs like XDCAM and even MagicYUV. Can someone else with an AMD card and VCE confirm my findings?
The only part AMD VCE is still broken is that you cannot change the resolution, only use the one that comes preselected. If you check the "use project resolution" the soft bypass it and use the preselected one.
So I still need to export to handbrake and render using VCE to use manual resolutions.