While creating a Shopping Cart, 'Vendor' Fild is non-mandetory field. This men th shopper can ceate and submit the requisition for approval, without mentioning the 'vendor'. The financial approver cannot 'edit' the requisition, as there is no 'Edit' button on approval scren ( though th instruction text instructs that 'edit' is possible')
Thus , the approver would have to 'reject' th requsition. The requisitioner then again have to populate the vendor and resubmit.
The concept o 'incompletion workflow' do not exist as Out O The Box Ariba solution ( like SRM).
Solution-
Mak vendor field as Mandtory. This would be a change in OOTB solution and would need a CRD ( cutom change by Ariba consultant)