Ok, while playing with this. I found the following problems:
Using pfn_touch ()
1) If player A drives his func_vehicle into player B, and player B never moves pfn_touch () never gets called.
2) If someone is standing ontop of the func_vehicle and gets killed by worldspawn a false "vehicle kill" will happen.
I also made a method that works via distance checks ( Basicly checks the distance between the the vehicles & the dead guy. If the distatance is bellow X and that vehicle has been used a "vehicle kill" has happed)
1) If someone is standing inside the distance of a func_vehicle that has been used and someone gets killed by worldspawn you get a false "vehicle kill"
Offcourse you can remove worldspawn kills that are triggered becuse a player died from a long fall. By hooking PreThink and checking if any players are airborn, and if he is. Check how long he has been airborn and if its longer then X Sec the plugin will ignore his worldspawn death.
But then again if someone changes the gravity on the server, it will all be messed up