You are limited to Part Number in VM as this is defined as the unique identifier in the Central Library flow. It would be better (though probably more work for you) to map your unique company part number to Part Number in the library. This ensures all of the tools will work correctly.
I understand why you have approached the library in the way you did with a generic Part Number (I also worked in this way previously) and there was an Idea on the DxDesigner Ideas site promoting this way of working but it didn't gather many votes.
Robert, thank you for getting back to me. Yeah that's what I thought. I also could understand the pros and cons of using very specific part number, or using our company number for part number. Fortunately I could use Part Lister to generate the BOM which is what I want, and don't have to use the export function in VM.