VT-40 has an 80% chance of increasing durability by 1 point each time the player takes damage regardless of the damage source. This results in it repairing armor by 1 point 80% of the time when the player takes damage from a source that shouldn't affect armor durability (e.g. fall damage and potion effects). It also means that VT-40 is much less effective than advertised in its item description when taking large amounts of damage that reduce a lot of durability on armor.