Can you give some insight on this business case?
you want make your vendor an offer? I thought you are buying from your vendor, usually he makes you an offer.
Or are you eventually a subcontractor to your "vendor" who is as well your customer then.
Wouldn't this process start in SD? you make your customer an offer, if he accepts you get the order, hence you create a sales order, and to produce this sales order you have to procure the components from your vendor.