I would need more information on timing and reordering rules to give recommendations for your specific situation but generally speaking I don't think it is possible to automate shipping a component to a company as a result of placing an order for a product with them. My recommendation would be to activate a warning for when you try to make a purchase order for Product A (ie. "Must send Component C to Vendor B before ordering this product!"). You can have a reordering rule on Component C to replenish stock from Vendor D when it gets low.

As for the valuation, I would create an invoice to sell component C to Vendor B and have them increase the price of Product A by that amount. You could avoid the extra transaction by just registering the SO as paid and paying them the discounted difference (what you're paying now). Either way I think you would have to create a sales order for component c to vendor b to track its movement and create a delivery order; the alternative would be to sell it for zero. 

A simple solution for valuation if you don't want to change the purchase agreements would be to change the costing method to Standard Price and set a price that adds the cost of component C.