Having VG's on the front splitter of the car will help organize the airflow.. like mentioned above..
but a full smooth underbelly/under tray like on race cars would be useless.. there is no flow separation UNDER the car except at the rear of the car.. which diffusers sort out.
Now air separates and exits out of the sides as well.. but VG's would be pointless on the sides of a car.. thats why race cars are so low or add side skirts.
In our case.. adding VG's on the front splitter for our Z's would help since our cars have a wild mess and lots of interference and parts sticking out...
and I suppose adding VG's on the bottom of the stock gas tank would help too, but nothing noticeable.