I am noticing that we purchase the same product from multiple vendors (depends on who gives the best price that month) and each vendor has their own part number. I am able to add the part number to the product record but that is going to change depending on who we select as the vendor.

The only way I have found that this will work for us is by creating a "product" for each part for each vendor.  That is going to be a lot of products for an IT department that does a lot of in-house repairs. 

I would think the ideal place to have the Part number would be on the Vendor tab of the Product record and then have the PO pull that number instead of directly from the Product details tab.