Vendor Primary Contact

Hi All

I have a vendor search which retrieves data just fine.
I would now like to get the vendor's primary contact also.
The documentation looks like you should be able to do a contactPrimaryJoin but does not have an example that I can understand.
I have tried all sorts of different ways to implement this but the search results never include the contact details.

I can do a separate contact search for each vendor and I do get a contact list but am then unable to determine which one is the primary contact.
There does not appear to be a direct link between the contact and the contact roles either.