We purchased a new vehicle a few years ago with the vat exemption.
Basically the delaer should invioce you vehicle with the VAT deducted at source.
You will need to complete a VAT exemption form and submit this to the dealer at the time of the purchase.
As Glos Guy said the vehicle requires permenent adaptions to be fitted such as hoists / hand controls etc a steering ball for instance would not count, though the guidance is a bit vague.