This applies to effects with an adjustable point graph, like VEGAS Distortion; VEGAS Vibrato; VEGAS Amplitude Modulation.

Trying to run projects made on VEGAS Pro 20.0 and earlier with those effects present gives me an error:

The user preset for Plug-in 'VEGAS Vibrato' could not be loaded. The default settings have been restored.

I tried troubleshooting with my friend, and eventually, I fixed it by replacing the VEGAS Pro 21.0 files that were dropped in "C:\Program Files (x86)\VEGAS\Shared Plug-Ins\Audio_x64" with the VEGAS Pro 19.0 version of these files.

So something changed in these DLL files which is causing this bug on older VEGAS Pro versions.