Hey! Was walking around the illusion and otherwise messing around, when suddenly my infinite dark vision went out. switching helms, taking off and on helms, or going without helms dosin't seem to work towards restoring it, but whatever the reason, just letting you know.
You can get it back by resting probably. These sort of things like to vanish when you log out and reapply when you rest. Need the helmet on when you rest, and rest again for a different helmet.
I think there's an hour duration limit. Just reapply the perk.
It is similar to the Lifestealer Vampiric Regeneration weapon perk, as it will fizzle after a lengthy time, and you need to rest with a weapon out to apply it to said weapon.
You need to rest with the helm on for the effect to be applied, but expect it to go away after a certain (lengthy) amount of time.