Seemingly only if the truck isn't being driven & has a gunner on the back the truck will bounce backwards from wherever you vault onto it from.

Video example:

Notice at 0:33 in the video, the moment he leaves the turret the vaulting produces no effect.
Also not in the video but when we were testing it out if someone is driving the truck it produces no result.
Also at 0:24 because of this vaulting effect it launches the other player backwards off of the hood.

So basically whenever the truck is parked with someone on the turret the vaulting the hood/bed launches the truck backwards.

Don't know if this is already known, just thought I should submit it.