Has anyone developed customizations for creating purchase orders from purchase requisitions to accurately be able to automate setting the amount of the PO to the estimated amount of the Purchase requisition, and automatically setting the vendor from the PR more easily without having to define the vendor on the item record?

So you fill in the vendor name in the Requisition transaction? And you want the estimated rate entered by the user in the Requisition to be a unit price in Purchase Order?

that would be ideal