VOX is crap on a bike. Set too low and you shout to get VOX to open and always have to remember to say something unimportant as the receiving radio will only get part of it. Pause too long for breath and VOX closes again. Set to high and one radio will stay on transmit and the user will probably not notice until he realised nobody is talking to him.
Get a PTT switch. The Autocom one has three position options via a toggle switch. 1 - Push to talk, 2 - lock on transmit, 3 - VOX. My wife and I have used them for a couple of years for European tours and they work great. Normally they are on PTT but if a long transmission is needed, especially in traffic where left hand is working clutch, it's easy to flick over to transmit lock. Occasionally they got moved to transmit lock by accident but now we have the radios on the handlebars and hey have red LEDs when transmitting (Alan HP450 which is IPX7 waterproof) this is not an issue.
They are intended to cable tie under the left grip which is great on my wife's CS with little hands and a wide grip. My GSA grip is a lot narrower and there is no room for it with my big hands so I use it on top of the bar between mirror stalk and clutch reservoir. A little fiddly but no real problem. As the switch has a drain hole on the bottom (now top) I drilled a new hole and stuck a bit of silicone seal in the original one.
PS
Just bought some new Autocom cables from Chainspeed - yes they are very helpful