I've never really had a problem with vibration on an eTrex (I've had 4 different models over the years), other than the rattling noise caused by the old style mount which is easily cured with a layer of tape. The eTrex 30 mount doesn't rattle. I don't believe that eliminating rattle between the unit and the bracket is going to make a significant difference to the amount of vibration it's subjected to - the bars on a rigid-fork bike are still a high-vibration environment, even if you don't have a reference to observe it against.
The spring contacts internal to my VistaHCX eventually wore their way through the copper layer to the non-conductive board beneath, at which point it became sensitive to hard jolts (actual potholes, rather than rough surfaces). Fixed it by soldering bodge wire between the battery springs and a convenient point on the board.
I do use a lanyard. So far, that's only come into play during actual crashes (on the recumbent, where I tend to land on the eTrex).
