I do not think vibration hurts a fire extinguisher. I wouldn't put it there, because it becomes useless. If you wanted it, I would assume something is on fire, so that means you have to get out, use your key to open the rear door, unscrew the cover knobs, (if you reinstall the cover) and then get the extinguisher out. I would think mounting it behind the driver's seat in the small window frame would be better, but I would only do that if I had tint, otherwise it might release the safety valve from the heat?

Here are a few pics of my sub install, I manage to fit tow straps, a tool set, and a first aid kit in there with room to spare. That is obviously not the original door cover (red knobs), it came off of an abandoned VX at a dealership in the area, so I still have a nice shiny new one in my shed