Planning run to consider Ware house stock before PREQ for Vendor

Hi All,

I need some help in setting up the system for below requirement :

1.When i am running my planning run at DC 1st I need to consider available stock at my Warehouse ( STO) and then create a PO for Vendor to fulfill any remaining demand .

2.While checking the stock at warehouse i need to check for Main product Stock and then check for Substitute Product stock as well before creating PO

Your valuable suggestions greatly appreciated for Prob1 or Both

Thanks in Advance,

Chandra