Callback NPC:OnVehicleStreamIn callback is called when a vehicle is streamed by the NPC. A simpler definition would be when the NPC sees the grey vehicle icon appear on his map.
Callback NPC:OnVehicleStreamIn was added in 0.3a and will not work in earlier versions!
For the player version of this callback, check OnVehicleStreamIn.

int vehicleid The vehicle that has been streamed.

Return Values:
This callback does not handle returns.

public OnVehicleStreamIn(vehicleid)
    SendChat("Great! I have just loaded a vehicle!");
    return 1;

Related Callbacks
The following callbacks might be useful as well, as they are related to this callback in one way or another.