Comparing Open Orders - What to use? VLookup within VLookup?

Hi, I've been trying to figure this out for a few hours using different ways but maybe there isn't away.

What I am trying to do is compare my open orders for a company vs the customers. Attached is the sheet with 2 tabs, labeled one for theirs and one for my reports. On their tab I'm trying to verify that they have it on order and our quantities match up. Since the same items are on multiple orders, is there a formula to find them within different PO numbers? I've tried researching this or trying to find a way to do it but have had no luck.

Thank you very much for any help or letting me know if it is possible.