I work in a Wireless device franchise store location, and I work with Smartphones and Tablets pretty much every day. I am somewhat familiar with the G3 in terms of usability and did discover that there is a hardware camera button feature. I haven't had the chance to verify it on the G Vista yet so I'm not sure if they incorporated it into the phone, but I can't imagine that they would leave it out.
If you have the screen powered off and you long press the volume down key on the back with your finger, it automatically opens the camera without having to go through the lock screen. And if you press the key again it snaps a photo. Perhaps you can verify that for me if you get the opportunity.