Well, the whole bike headlight and battery came together as a complete system so I'd have to trust the vendor put the parts together appropriately. As has been pointed out above, you can pack more energy into a Li-Ion battery than any other rechargeable battery--a reason y'all like them so much. For a bike, that means the battery can be smaller and lighter yet deliver a lot of energy over time.
I don't think vibration has anything to do with it (my battery actually sits in a pack and is not directly connected to the bike frame)...it is the chemical make-up.
Let me put it this way, I paid boo-coo bucks for this nice light system and I like it a lot because it is very bright but there are several disadvantages with it I've noticed over time:
(1) The bulbs don't last long and are proprietary (near as I can find) and are therefore pricey.
(2) The battery doesn't last long and I'm not sure even when brand new that I ever really got the expected capacity out of it. After reading PrimeCells page on Li-Ion batteries, I think I understand why: Li-Ion batteries appear finicky.