[Imp]

Only option would be to slow them unless you are going to make vehicles viewable some how above the global visibility settings.
Spotting vehicles is not effected nearly as much as spotting infantry in low visibility games as a general rule.
Just like a tank firing would at the very least give away its position due to the flash but doesn't if above the global visibility setting.

A nice touch in my view that MAY be fairy easy to implement would be an option for the global visibility to randomly adjusting by a hex or two each turn.
Something like 5% chance plus X% per turn its stayed the same

Moon appears from behind the clouds, thunderstorm, blizzard eases up etc.