Sorry to say, the NACA duct doesn't do anything useful on the Viper -- a regular old opening would do just as well. Basically a NACA duct's purpose is to scavenge boundary-layer airflow without introducing additional drag. It does this by gradually creating a low pressure area using a ramped intake aligned with the airflow. The diverging sides of the duct simultaneously create small vortices and reduce how close the duct walls are to the air (the "charge") flowing into the duct making it harder for the charge to escape back out over the walls. The curved shape was basically determined to be optimal through plain old experimentation (in comparison to parallel walls).
In the case of the Viper's hood, the duct "hits" the air head-on, so there isn't anything for a NACA duct shape to do. There isn't an undisturbed airflow to coax a charge out of at that point on the car's surface. In fact, the curving sides of the duct walls simply create interference with what would otherwise be direct airflow into the duct opening, so the Viper's duct potentially does more harm than good relative to the opening size.
Although it does look damned cool and I wouldn't trade it for anything.
Why do I know this much crap about NACA ducts? I just happened to finish doing a lot of NACA-duct-related work last night, oddly enough. Mostly based on the original NASA study from 1947, which you can read at the URL below (funny to think it used to be classified material) :
http://naca.larc.nasa.gov/reports/1948/naca-rm-a7i30/
It's cool -- take all those calcs and figures, plug in your site requirements (max depth, run length, etc), and voila, out pops a NACA duct.