Various unit traits/abilities govern the sort of vision their bearers have. None of these are stat buffs in the typical sense; rather, they counter various debuffs, or buffs on other units that act as debuffs for their attackers.

Darkvision reduces the penalties of Darkness. In Caves and Deep Seas, the Sun's rays can't directly reach the battlefield, resulting in penalties to Attack Skill, Defence Skill, and Precision. These penalties also apply at night, while certain spells can apply weaker or stronger penalties. The point value of Darkvision, either 50 ("Low Light Vision") or 100, is a percent decrease to these penalties.
Nature Alterations can provide 50 points of Darkvision, as can the Low Light Vision Bless.

True Sight is magical vision that lets a unit discern real things from unreal things. The purple contact lenses will filter out mirror images of elven or elf-aided foes, along with annoying distractions such as Displacement and Blur. It also corrects the "perception filters" of Invisibility and Dread, and it improves vision just enough to counter Twilight. It is not Darkvision, though.
4 mages always have True Sight. Glamour Enchantments can provide True Sight, as can the True Sight bless.

Spirit Sight and Blindness are, effectively, Darkvision 100 and True Sight combined. The former works by allowing a unit to see the essence of things, while the latter simply works by forcing a unit to stop relying on their eyes. Spirit Sight is an Astral/Death Enchantment and an Astral/Death Bless. Blindness is typically a debilitating Affliction, but some units are born Blind and have adapted to it.