I found out from experience at the ring that if you spawn your vehicle inside pvp while you are outside pvp,
people in pvp would not be able to damage your vehicle.
Also after you entered pvp,
they still will not be able to unless you enter the vehicle.
After that even if you alight from it, they still can destroy it.
PS: Your vehicle doesn't decay if you take environmental damage(rolling off mountains, bumping into trees/invisible ojects). Only your structural integrity goes down.