Hi,
In that case, please check whether subcontracting scenario can suit for the business process. Create BOM for the material supplied by your vendor with component as the critical part. Your PO to your vendor should be a subcontracting PO with the material.
Create a free of cost PO for the "critical part" for the the second vendor and maintain the SC vendor in Delivery tab as the 1st vendor. Once you get tracking invoice, do GR for this dummy PO, which will post the GR to the special stock of the 1st vendor. Alternatively, you can find other options to post the critical part as vendor special stock.
Now, you can receive the main material against the first subcontract PO.
You Check the option and revert back for clarification.
Regards,
AKPT