It's hard to give you a quick answer because you didn't tell us what version you are using. So a general answer is:

If you are using Quantum, then activate vendor specific inventory, enter the vendor's ID for each part and tell the program to use it on the PO's. Then you can use your internal ID for your own sales invoices.

Otherwise, you will have to do workarounds. I would continue to use your own number for sales and purchase invoices and orders. However, I would customise the vendor PO to use one of the Additional Info fields for the Vendor's item code and enter that for each item in the appropriate field. You must enter something in every item's field and you won't be able to see it on the data entry screen but you can force a pop up to show you the code if you wish during the inventory item setup on the Additional Info tab.

Sorry it's sage 50

Thanks but this is a Sage 50 forum, so I assumed that or Simply Accounting. I was referring to Pro, Premium or Quantum and which year because each has different features available as you get to the higher end versions. Different features means possibly different solutions. If you were using First Step, then you can forget about managing inventory as it is not a feature of that version of the program. Hopefully one of the answers in my last post will help you.

whoops - sorry - Pro

Then using the item Additional Info fields is your better option I think.