Optimize preferences: take effect when save is pressed, rather than requiring restart
Where possible, don't require the user to restart VLC before changes take effect. Use the "save" button to trigger the changes. For settings where this is unavoidable, add a warning dialog when "save" is pressed that "Some changes will only take effect after VLC has been restarted". Also add a warning when cancel is pressed "Are you sure you don't want to save your changes?" yes leading to cancel, no changes are saved, no changes take effect, and the preferences dialog closes. No only closes the warning, the preferences dialog stays open, but the changes are not saved yet nor do they take effect until the user chooses to save.
For the sake of an intuitive user experience. This is at least consistent with Windows user experience, but probably also desirable on other platforms.