Hello,
As far as stdn SAP QM practice is concern, stock posting is through lot only from QI to UNRES or QI to BLOCk etc. So already one lot is there against this rejection qty and document generated in bkgrnd.
Now, your query is vendor first rectify and from Block to QI and then QI to Unres then you need inspection lot for RR and UD so that if it is ok then move to UNRES and if not then remain in BLOCk stock.
Now, Via mb1b when you post the block qty back to QI via 350 there is off course inspection lot and via QA11 only you can do RR and UD and stock posting based on decision.
as per your query, now your need is from same screen no mb1b, better to go with notifications and its action boxes. create notification in backgrd and assign FM for transfer posting in action boxes. from Blck to Qi this help to generate one more lot and from there you achieve your requirement.
Regards