I presume that your PO with vendor B is subcontract PO where-in consumption of Engine is working fine, but the component C is not working fine hence this issue.
Does your BOM includes Component C, if so how the related inventory is handled. Provided inventory of Component C is not handled by your company then this is typical tolling process solution for which SAP doesn't have standard solution. So you have to go either pick Sol-1 or create a user exit solution to get your account postings correct.