For a darker, high-contrast vent:
Use small-sized textlights made pretty bright-->500 depending on how big you make the brush. This gives you a really-dark space with small, really-bright areas.
For a brighter, low contrast one:
Bigger-brused textlights with a lower brightness value. Gives more even, yet more bland/boring light.
If you're going for realism, there wouldn't be any lighting in the vent at all, and the only "natural" light you'd get would be through grates, or damage to the duct like it was crushes by something, bullet holes, or explosions.