Hi,
Even if you go for user exit, may I know how the system knows which items vendor is going to deliver? - You have to select it manually.
If you use VL31N, then you may click the "purchase orders" button, which will allow you to select the PO number and item numbers and then select the items to be adopted. Else, in the inbound delivery, you can maintain the put-away quantity only for the items which vendor is going to deliver, delete the other items and then post the GR. Only these items will be displayed in MIRO if its GR based IV PO item. If its not a GR based IV item, then you may search in SCN, how to remove these items in MIRO.
Additionally, you may use VL34 transaction, where you can maintain the PO item number as well, and inbound delivery will be created for the specified item only.
Regards,
AKPT