Currently not all devices are optimized to handle Fog of War, and for these devices we default them to a lower graphics setting, which disables it.
For devices with lower specs GPU, the "Auto" option in Graphics Quality Settings will default the game to a lower value, as it helps optimise your device for a buttery-smooth Vainglory experience. The Fog of War will be activated in all its (Vain)glorious splendor when running the game with High Graphics Quality settings.
If you'd like to experience the Fog of War during your 5V5 matches, you may want to change your Graphical Settings to High, however, there can be a bit of a catch with lower-specs devices; when running vainglory in higher Graphics settings, your framerate is likely to take a hit and impair your ability to be fully and truly awesome during your matches.
If you experience any type of less-than-buttery-smooth behavior when running your device's graphical abilities to a higher value in Vainglory, we would suggest that your lower these from the game's settings.