What happens when you see someone, draw your weapon, and suddenly a smoke grenade explodes, or the room goes dark? What penalty needs to be applied to a Dex + Firearms roll? Is it the same as blind fighting? What if you have no idea where your opponent or your friends are in subsequent turns?
-1 to attack enemy whose position you can accurately approximate. -3 in subsequent turns.
Describe combat scene in terms of abstract shapes and silhouettes. ALL combatants are fair game. The GM should keep a mental picture in their head of where everybody is. Confuse PCs through vague descriptions.
To figure out where allied/friendly PCs are, have them make [Presence + Wits/Expression] vs [Wits + Composure]. Exceptional success on either side equals exact position now known -- before then it's all relative.