You have the emiter [E] of the particles and you have the vehicle [V]…the you parented them:
[E]-----[V]
Its fine…should work perfect…EXEPT…if that vehicle has physics. An IPO motion cannot work at the same time with a PHYSICS driven parent.
So if your vehicle has physics associated with it…I don’t think you can run both at the same time.
THere is a workaround…use constraints to keep the particles tied to the vehicle while still being able to perform the IPO. Hope it helps.