My customer has tested Mechanical switches hooked up behind the lift pedal, and liked the solution. They used a serial connection to the VC. Look up Vehicle In-Motion Detector in the manual for a description.

I was on the original design team for the VC5090. We designed in a way to blank the screen based on truck movement. One of the Engineers should remember this, Richard Yost and I spent many hours making sure this was added. While I do not remember the exact technique, I do remember if the unit sensed signal on one of the Serial pins it would assume the unit was moving and blank the screen. I seem to think they tied a servo into the fork truck gas pedal, but could be wrong on that. Sorry, but that is all I remember. I am sure it is documented someplace.

we usedto have a Motion Detector Cable, but it went obsolute late in 2011. Not sure if you can still get any, sometimes they are keen to sell off the old stock or perhaps a partner has some left. Look in the PRG for how it works / is installed, you maybe able to get a thrid party solution to do the same thing....it was part number.... 25-82420-01R VC5090 Motion Detector Cable (without motion sensor switch)