There are couple of settings that you should check in VLC:

1) Tools --> Preferences --> Show All --> Video --> Disable screensaver

2) Tools --> Preferences --> Show All --> Advanced --> Inhibit the power management daemon during playback

If both of this options are checked, you could try a simple wrapper script which disables the monitor modes that will power down, standby or blank the screen prior to calling VLC:

#!/bin/bash



#Disable modes

/usr/bin/xset -dpms &

/usr/bin/xset s off &



#Open VLC

/usr/bin/vlc "$@"



#Re-enable modes

/usr/bin/xset dpms

/usr/bin/xset s on

exit