This is not currently possible directly.
However, you can use a workaround by enabling Must Require A Receipt on the Expense Type, then creating a VAT Receipt Option with Allow Receipt Override enabled.
Create a new receipt type called 'No receipt' and select the Allow Receipt Override checkbox.
Mark the existing 'I do not have a receipt' VAT Receipt Option as inactive.
When a user uses this expense type and selects the 'No receipt' option, they must enter a comment instead of uploading a receipt. If they select a receipt type that does not have Allow Override enabled, such as 'I have a VAT receipt', they must upload a receipt.