Hi,
I would suggest you to create a separate PO / line item for the component for the vendor with the required BOM qty. In item level - delivery tab, maintain the vendor name as well as "sc vendor" indicator. Maintain the price in PO if the vendor is charging it separately or maintain it as "free" item.
Now, do GR for the PO before GR of the subcontract PO. The movement type will be automatically 101 O , so that the goods will be posted to subcontractor stock. After GR, if you check the stock of component B, it will be shown as "stock provided to vendor".
After that, do GR for the subcontract PO and proceed as in normal case.
Regards,
AKPT