My tuppence:
Vibration will not affect hard drives muchif they’re not running [seriously, check the rated allowable maximums, it’s like 100G when off]. However, if they’re running I would imagine a single bumpy/vibraty ride would kill one.
Therefore, if you can’t afford/don’t need a machine with an SSD – make jolly sure the machine can’t wake up through the mouse/keyboard [wireless mouse in bag would trigger, squashing the thing and hitting a key would too].
More of an issue, IMHO, is squashing the thing. My friend has been using a wafer thin silicone cover over the keyboard for the three years he’s had his Macbook, and unlike almost every other screen on a laptop I’ve seen, there’s no key-marks on the screen. Usually if the laptop is closed and squashed the keys will hit the screen and leave nasty scuff marks.
Apple, and some other manufacturers have had major issues with solder on logic boards dying. Vibration would hasten this, and it’s the most expensive or difficult thing to replace. However, squashing/warping the machine would cause this to happen much faster!
But, yes vibration will eventually do for the things, my NC10 survived 9 months of touring, but when it eventaully died, disassembly revealed the heatsink had worn though the CPU die and killed it!!
If I were you I’d be looking to use either a good pannier [make sure it can’t fall off] plus a neoprene sleeve. Or, a backpack with very little else in it [your back is curved, the laptop will want to bend around it unless you pack very carefully]. The other issue with a backpack is sweat.
Backpack – less vibration, more squashing [potentially]
Pannier – less squashing, more vibration.
There is no easy answer.
Ortlieb roller-tops worked well for me.