I will take a shot at it. Although I am not a Technically very sound. The changes to PO are stored in the below tables
You need to write a wrapper code with change classification types which will fetch the nature of the change by running a comparison between base tables and archive tables.
Based on the change classification code from this wrapper control the behavior of the Workflow attribute which notifies vendor on final approval of the document.
Let me know your thoughts.
Thank you so much Abhi for your inputs. We will try this and get back to you if we have any queries.