All I can suggest is Hex Editing through trial and error. Even then you might not get anywhere.
You can double tap "E" to exit a vehicle quicker. This will leave the engine running. It's a Vanilla feature. I believe it's hard coded.
I'm not sure if this would work, but if you were to remove all the regular exit animations from the Vehicle's Animation Set, maybe they would be forced to use the quick exit. You can see my
Animations guide where you'll find a tree of all the animation sets for Vehicles.