Have you investigated using a third party subcontracting (or direct subcontracting as it is called) in SAP. The process flow is like this:
1) Sales order from customer A
2) PR from SO, Subcontracting PO from the PR to vendor 1
3) PRs from Subcontracting PO for components, convert to POs to vendor 2 and 3
4) GR of components, components transferred to vendor 1 for processing
5) vendor 1 sends goods directly to customer A.
Check out this docu, hope it helps simplifying your process:
