I've actually seen that, but it confused me more. Aren't customer invoices saved stored in account.invoice as well?

Yes both are saved in the same model, ie account.invoice , customer and invoice are distinguished by the field named type

*customer invoice and vendor

I only want to use it to add/edit vendor bills just like you can add/edit customers on the Customer Invoices form. How do I refer to the vendor form?

You simply fill in the field 'type' with 'in_invoice' - which means it is an incoming invoice (vendor bill)

Great, but where's the documentation for things like 'type' and 'widget' fields? I added the the 'type' field, but the area still appears disabled probably because I need to define the 'widget' for dropdown, and I don't know what's the widget string for dropdown with add/edit.

Okay, it's something else. In account.invoice.line, I made a many2many field with two models (to try it out) account.invoice and account.payment models. Both appear as grey uninteractive fields. I think I should define some identifying strings from the models, but I don't know how.

I was wrong, I needed the many2one field. Is there anyway I can make it display the vendor reference instead of the autogenerated bill no as the string?