[Imp]

Fail to see the logic here being a helicopter or a ground unit makes zero diffrence to the visibility rating the extra height of the helo does make a diffrence to its fields of view as it can see over stuff the ground unit cant.
What that means is it can make use of its visibility rating far better than the ground unit because any obstacle blocks the view.
Visibility ratings & fields of view are two entirely diffrent things land the helo & it can now see what the comparitvly equipped ground vehicle can see.
Stick the ground vehicle on a level 5 hill & it now has better fields of view than the low flying helo thats now below it skimming over level zero ground.

Yes the helicopter with vis 0 has a blanket over it if global vision dictates, moonless night or fog if you want a blanket, its likely to find a hill abruptly before it finds a target if it tries NOE.
If it goes high its a prime target for any ground unit with vision aids & AA capability.