nVidia playback problem
Using nvidia-xconfig with Debian, the screen may go into standby mode.
This doesn't seem to be a bug with VLC media player, but it may occur when using VLC, such as when watching a movie.
The easiest thing to try is disabling the screensaver through Preferences → Video → Disable screensaver.
Another approach is to run
$ xset dpms 0 0 0 && xset s noblank && xset s off
which disables DPMS (Display Power Management Signaling).
If the issue does not resolve, and it seems VLC-specific, then report a bug.