The VFD is held away from the display board by some foam pads which under shock (ie dropping it on the floor like I did!!) allow the VFD to move closer to the board. Immediately beneath the fill nipple is the body of the volume control. The nipple hits this and breaks off. I know this cos I broke mine this way and on close inspection I could see bruising on the volume control from where the nipple hit it.

I think the only sure way of preventing this is to put something more solid behind the VFD to reduce it's movement.