From the screenshot you've posted, it looks to me like it actually captured the window, but the INNER CONTENTS of VLC Player window are black.
That suggests to me that the problem is not related to any security issue, and that nothing changed in your screenshot files -- they were almost certainly always black inside the VLC window.
Assuming I'm right, the first answer to your questions it that there is nothing you can do to restore those images.
The reason for that is that some video players (VLC) and games will use an ACCELERATED video mode to play video at the fastest speed, which can interfere with screenshot taking tools.
There are a few things to do going forward:
- First, do your testing on VLC while playing a movie and make sure your screen captures show the contents -- since normal screenshot capture may work perfectly on everything EXCEPT the contents of the vlc window.
- See if capturing an entire screen with Screenshot Captor bypasses the problem.
- VLC has its own built in screenshot tool you could use (at the bottom of the Video menu).
- Disable accelerated video mode on the adapter (or better yet inside vlc -- though someone else may have to help with that since i don't know if there is a setting for this); the downside of this is that it may slow down playback but testing will tell you that.
Do let us know how it goes.
There are some cases where a capture is ENTIRELY black, which are caused by a security program being an ass; but SC will detect and warn about such cases. In your cases unfortunately you can see that the controls of the window were captured just not the contents, so that fooled SC into thinking the capture was successful. One thing I could do going forward is check from a list of known applications being captured (vlc, etc.) and warn on the first capture with some tips..