GSX will always sent the correct vehicles for the amount of passengers calculated. The small van is supposed to appear ONLY if there are less than 5 passengers on board.
The only possible explanation for this, is that you (or the airplane itself), changed the payload *after* GSX already sent the small Van, because it calculated a very low amount of passengers because of the payload being basically empty at that time.
If the payload changes after the passenger bus is already on its way, it won't be destroyed and recreated with the proper model while it's arriving.
Either be sure you don't call GSX before changing the payload OR disable the "Estimate Passenger Number" option in the GSX Settings, so you can confirm or change the amount of passengers at any time GSX requires to calculate it.