This may be Windows-specific.
If I pause a playback in VLC, leaving the application window open, Windows (Windows 7, not sure about others) fails to detect inactivity which would normally cause the system to suspend, given the way I have configured it. I can explicitly tell it to go to sleep, and that works. However, that is not sufficient. If the computer wakes itself for a scheduled operation (like recording a TV show, or downloading TV listings data for Media Center), then it does not go back to sleep after the scheduled task completes if a VLC window is open. Thus, if I am in the middle of playing a file and wish to resume on a later date, I cannot simply pause the playback to maintain my position in it. Instead, I have to close the program, or else auto-sleep will not work if the computer awakens later for any reason. This is a nuisance, since I must make note of my position in the playback so that I can get back there when I wish to resume viewing.
If VLC could remember its last playback position in a few recently played files, that would also help a lot; but it would be better for me if VLC did not subvert the detection of inactivity.
I have not encountered any other application which, when left open, subverts Window's ability to detect inactivity; so I would not expect this to be all that difficult to fix. In fact, expecting that this problem would surely get addressed, I have gone through at least 5 upgrades, only being disappointed to observe each time that the problem still exists. I regard it as a bug; but, given the present situation, I would regard a fix of the bug as an added feature.