Not sure about additional vibrations but it makes sense it would have some...
FFT
If you mount it to the dash pad you have the dash pad supporting the Zumo only...The dash pad is held in place HOW?
It is held in place by a couple potted fasteners thru the inner fairing...
Now what happens when the potted fasteners fail or crack the plastic that is holding the fasteners in place on the back side of the dash? You beautiful painted inner fairing could possibly get damaged...Now along those same lines when the fastener fails on the zumo side and your touring down the highway and your unaware that it has failed and now the additional vibrations and weight becomes to much for the one and only fastener on the other side and it fails...

Got the picture yet...
FFT
Be prepared for more vibration of the Zumo's screen when mounted to the bat wing dash vs mounting to the handlebar. It's also a longer reach which in the end you may find is less convenient that having the Zumo mounted to the handle bar.