By VAT rate, did you mean VAT tax?
Are you implementing 12.1 or 12.2?
In release 12 and above, you will need to have Oracle E-business Tax module implemented.
Tax can be entered or update using Tools > Manage Tax from the core requisition form.
An overview of EBusiness tax is here:
Overview of the R12 E-Business Tax (EBTax) Upgrade for Procure to Pay (Doc ID 1362208.1)
Hope this helps.
I used the document you highlighted to change which fields are visible and can now see the option in the billing tab in iproc. Also managed to remove some other unwanted fields. Thanks.