Night Vision Potions cause night to appear day for the drinker, which makes seeing a lot easier. They have the same effect as the Night Vision spell. Since many prefer the day to the night, players often store Night Vision Potions in large quantities for future use. This potion normally lasts all night. However, logging off the Oberin game immediately cancels this potion, so if you log off and then return later that same night, it will appear as night again.
There is also a quest involving Night Vision Potions: trading 10 bottles to Braask Boolin at the Strangled Troll Weapon Shop in Lerilin will earn you 10 Experience Points and a Dagger of Durability. And as with all potions, they can be bought in the Potion Shop, and only up to twenty may be carried at one time.