There is no need for development, vendors (and customers) have a standard ability to be blocked for payment, using sensitive fields, the vendor can be blocked for payment every time a sensitive field is changed. this also works when a vendor is created.
It is also possible to release the vendor block via transactions FK08/09. you can of course use workflow to direct the approval to the right approver. but this is not a must.
see this link for more details
It is also possible to raise an event to start a workflow using change documents in this case using the CONFS field in LFA1 or LFB1 tables. so you don't need to use enhancements to start your workflow.