learly I am missing something fundamental. I have imported a bunch of vendors, with external IDs. The do appear when I export them and when I look in the External Sequences section. I am not working on loading a few products with the Vendors, but I am getting the error: 

    No matching record found for external id '110388' in field 'Vendor' at row 81.

There is a Vendor with that External ID. I am using the 'seller_ids/name/id' column heading which maps to Vendor / Vendor / External ID, so I am assuming this will work.

Most of the documentation refers to starting with an export as a template and then using that, but the interface doesn't seem to indicate what fields are required. Although I don't think that is the issue.

I know that my External IDs for the Vendor are not very unique. I can probably amend them, but I want to use some part of them, since this data is coming from another system.

Could someone give me some advise about how to approach this.

In general, I am not finding the documentation on Odoo to be very detailed. General Google searches give me more information, but many of the tips are for openerp 7 and I am not certain of the differences.