[PrO.GameR]

Simply because you shouldn't send a message when a player isn't even close to a vehicle when looping, right place to send that is outside the loop, if everything fails.

PHP Code:

for(...)
{
        ...
}
SendClientMessage(playerid, COLOR_GREY, "You are not behind any van."); 


And you also need to return when finding the right vehicle to load cargo behind, so it won't keep looping.