Are you sure you need a van service for this trip? The standard cab fare from the Bs As pier to Recoleta/Palermo/Puerto Madero hotels is about 20-25 pesos or less than $7. Five regular metered, honest cabs ought to fulfill your needs although the cabs at the pier tend to be neither. You can always call a radio cab company to ask to them to send as many cabs as you need to transport your group to their respective hotels. The phone numbers of the largest radio cab cos are:
Radio Taxi del Plata 4504-7776, 4505-1111
Radio Taxi Pidalo 4932-2222, 4956-1200
Radio Taxi Premium 5238-0000, 4374-6666
In case you really want a minivan (15 passenger) you can try:
http://www.adroguebus.com.ar/adroguebus/home.htm , but it seems this would not be a better option just to go from the pier to your hotels.