This has been Released. To enable select 'New design' from the Dashboard >> go to Suppliers >> Within a Supplier go to the 'Options' tab and select 'Enable default VAT rate' (note to add a new VAT rate not visible in the drop down go to VAT settings in the Settings section)
I have also asked about this some time ago as we do business with financial advisors who are not VAT rated.
I also agree as I deal with some suppliers who are not VAT registered, this would be a big help!
I had just filed a similar request (now deleted) as this is exactly what I suggested too.
Not all suppliers use the Standard Rate VAT. Some suppliers only supply goods or services that have 'Reduced Rate' or even 'Zero Rate'.
Currently, when adding zero rated train tickets for instance, you have to manually change the rate from 20% to 0% for each line item and again for each purchase.
If the Supplier setting had an option to set the default VAT for that particular supplier to something other than Standard Rate not only would it save a lot of time, it would also prevent many erroneous VAT entries.
I asked support about this yesterday and they said it was a good idea and I should go to Canvass and add this suggestion!