You should give each side some sort of vision modification that they can use (possible that uses up some sort of energy.)
For the slayers, night vision that lets people see in better contrast and also makes vamps easier to see, especially when they're invisible.
For the vampires, some sort of thermal vision would be nice, possibly that lets you see through thin walls, kind of like in the hidden: source.
I just think this would add a little more depth to the gameplay. Also, an alternative to using energy would be making it so that the modifiers aren't entirely beneficial - for example, maybe thermal vision makes it so you cant see very far regularly, and night vision makes it impossible to see in well lit environments/in view of street lamps, etc.