well I'm guessing that just the ground vehicles get disabled cause if a jet or chopper gets disabled it'll just drop and crash but I don't know maybe that is the way it works but you can still insta-kill vehicles you just need to do enough damage in one hit to take it down to zero. Say a vehicle gets disabled at less than 30% health and you do 40% worth of damage you'll insta-kill it if it's health is low enough but if you only do like 20% damage it'll get disabled instead.