To create an invoice or receipt with 100% VAT, go to Sales > New Invoice or Purchases > New Receipt. Add a line to the invoice:
- Quantity – 1.
- VAT Rate - anything other than "n/a".
- VAT Amount - the value of the VAT.
The VAT rate you select in the drop-down box is only used to automatically calculate the VAT amount for you so by following the above example you're just overwriting the automatically calculated VAT amount.