The current vibrance does not reset to the value selected at "Windows Vibrance Level" when you exit VibranceGUI while having an Program/Game open that you have selected to use VibranceGUI with.
Example:
- Affect Primary Monitor only" is set.
- VibranceGUI is on another Monitor
- Program to trigger vibrance change on is located on primary monitor and has the current focus.
- Vibrance changed.
- Close VibrangeGUI -> Vibrance does not reset.
This is just an example of how to reproduce, but like anything that does not trigger the Windows Event (Hook) or triggers it but from a Program on another Monitor while "Affect Primary Monitor" is set should lead to this issue.