1. You can create a standard PO for vendor X and give the delivery address as vendor Y and tick the SC vendor.
Then when you post goods receipt, then stock will goes to directly vendor Y..
You can check the stock from MBLB.
2. Then create another subcontracting PO with qty 80 MT, and give the component of 100 MT.
3. When you post GR, your 80 MT will receipt in plant A and 100 MT will consume from vendor Y.