This won't work, first, getVehicleID is a deprecated function, second, you're defining the vehicle without checking whether the killer is a player.
This should work i guess:
local reward = 200
function kill ( ammo, killer, killerweapon, bodypart )
if ( getElementType ( killer ) == "vehicle" ) then
if ( getElementModel ( killer ) == 425 ) then
local controller = getVehicleController ( killer )
givePlayerMoney ( controller, reward )
outputChatBox ("#ff8800[REWARD] #ffffffYou have killed " .. getPlayerName ( source ) .. " and rewarded $" .. reward, controller, 255, 255, 255, true )
end
end
end
addEventHandler ( "onPlayerWasted", getRootElement(), kill )