Blanket and Standard Purchase Order

Can anyone help me with the Concept of Blanket Purchase Order and Standard Purchase Order. We have a Central Vendor for supplying Medicines accross the country. Our users are also scattered accross the country.

For the next fiscal year we want to change the supplier and have to call for quotations from other vendors. We will finalise the vendor and agree upon the rates at which the medicines will be provided.The quanity however will not be finalised at the beginning of the year,only the rate will be finalised.

How do we go about the process. Should a requisition (using shopping list) be created and then RFQ, Quotation and Quote Analysis be done and  a Blanket Purchase Order be created.