The M$ HEVC codec would possibly only affect those created files. The other ones should have loaded fine.
Since things aren't "working" for you now, you can try using Revo Uninstaller Free to uninstall VP16. I've used it in the past when regular uninstalling didn't work with other software, Vegas has been no problem. But it looks for leftovers in the registry and leftover folders that are often missed by using M$ uninstall.
Make sure you have your activation number ready.... and you do this uninstall at your expense....
Maybe it is a Windows security problem. When Vegas has to activate a codec that has not earlier been used and Vegas has to pay for the use of that codec, your hardware on which Vegas is installed has to make contact to the internet to load that codec. So look into your Windows settings and if needed allow it.
What is the name of application that does Vegas codec activation. It is maybe firewall causing this problem. What happens when I have Microsoft HEVC codec in PC. I disapled firewall but no succes activating. Vegas 10 pro works fine all the time.
I have reinstalled Vegas16 and Vegas21 trial version again, but nothing helps toV16 codec activation. V21 trial works correct. I installed Vp16 and Vp21 trial version on the other PC and both work correctly. This is incomprehensible. I also cleaned the register carefully from time to time. No C:\Program Files\VSTPlugins folder in my PC.