1. You need to you use a workflow to set the approval status - in your case just one workflow action to set the field to approved.
2. By design requisition doesn't have vendor specified at the header level, because you can create a requisition without a vendor or specifying at the line level. Now at the line level you can have different vendors for each line not nothing. When procurement goes through the the requisitions they can assign/source vendors by line level, even consolidate multiple requisitions.
3. you can add a custom field (employee) at the line level to populate with requestor (employee) - which will carry over to PO and use that value to trigger your PO approval process. Better option would be to get the approval at requisition level so no need for PO approval, this will facilitate consolidation of requisitions by your procurement team. Scenario if you 5 different reqs going to same vendor then your procurement team can consolidate it to one single PO, other scenario is a requisition can have request for purchase multiple items from various vendor and you can create PO for all those vendors separately. These are the best practices of P2P process.