The VOX headsets you are talking about are good for normal use but tend to get kind of annoying when in a loud environment. Excessive noise will cause the channel to remain "open" even when you are not speaking. It would probably work good for you, since you are in the cab, but most likely you will be getting "false" transmissions from your workers around the machine. Not saying it won't work, but even the ones with sensitivity settings don't like loud environments.
A better solution would be a 2-way radio with a shoulder microphone. The shoulder microphone plugs into the side of the radio and is clipped on your shirt or vest. This lets the radio remain clipped to your side and the microphone easily accessible. Still requires you to key the mic with your hand but you don't have to have anything in your hand, or worry about clipping something back on your belt, after you are finished talking.
We use these here at the quarry.
Kenwood TK-3200
They are pretty weather resistant and hold up really well in our quarry environment. They will set you back about $200 for the radio and $50 for the shoulder microphone. Let me know if you needs further comment.
On Edit: I had forgot about the head sets like Lash posted. I've never tried them personally.
I should add that I have tried the VOX before, while moving our trains, and we had problems with the mic staying open due to the noise level. This was the Kenwood VOX system that we tried.