I would like to create a custom field in Vendors, for example, a Fax Number. I would also like this Fax number to appear on the header of the Invoice. I assume that I will need to :

1. 1. Modify the database by adding a new field in #__invoicing_vendors table.
2. 2. Modify the Model by editing administrator/components/com_invoicing/models/vendors.php to include the new field.
3. 3. Modify the View by editing administrator/components/com_invoicing/views/vendor/tmpl/form.php
4. 4. Modify INVOICING_TEMPLATE_INVOICE template to include a fax number tag (for example {vendor_company_fax} ) in the header.
5. 5. Modify the tag replacer by editing the replaceTags() function in administrator/components/com_invoicing/helpers/format.php to fill in the fax number tag with the record from the database.

Am I on the right track so far, or am I still missing something? Any help will be most appreciated.

Yep, it works fine. An extra step I have to do is to include the "Fax Number" tag in the en-GB.com_invoicing.ini so the line will read

[INVOICING_VENDOR_FAX] {vendor_company_fax}

in the header.