Semi-duplex has no advantages to this service. But there are ham systems that are set up as full duplex systems that cover the entire western United States, and you can add as many full duplex users as there are inputs. But I digress..
So you either have VOX or you have full-duplex, there is no reason whatsoever to have both. The problem with VOX is that the transmitter keys up without notice from background noise, especially when standing next to the engine of a DP.
I'm really not sure what the complication is. Grab the FRS and use it like an HT just like countless millions around the world.
(Edit: iphone spelling)